diff --git a/js/usercss.js b/js/usercss.js
index 551fcd39..7e2f41ad 100644
--- a/js/usercss.js
+++ b/js/usercss.js
@@ -46,6 +46,49 @@ var usercss = (function () {
     }
   };
 
+  const colorParser = (function () {
+    const el = document.createElement('div');
+    // https://bugs.webkit.org/show_bug.cgi?id=14563
+    document.head.appendChild(el);
+
+    function _parse(color) {
+      const [r, g, b, a = 1] = color.match(/[.\d]+/g).map(Number);
+      return {r, g, b, a};
+    }
+
+    function parse(color) {
+      el.style.color = color;
+      if (el.style.color === '') {
+        throw new Error(`"${color}" is not a valid color`);
+      }
+      color = getComputedStyle(el).color;
+      el.style.color = '';
+      return _parse(color);
+    }
+
+    function format({r, g, b, a = 1}) {
+      return `rgba(${r}, ${g}, ${b}, ${a})`;
+    }
+
+    function pad(s) {
+      if (s.padStart) {
+        // chrome 57+
+        return s.padStart(2, '0');
+      }
+      return `00${s}`.slice(-2);
+    }
+
+    function formatHex({r, g, b, a = null}) {
+      const values = [r, g, b];
+      if (a !== null) {
+        values.push(Math.floor(a * 255));
+      }
+      return '#' + values.map(n => pad(n.toString(16))).join('');
+    }
+
+    return {parse, format, formatHex};
+  })();
+
   function getMetaSource(source) {
     const commentRe = /\/\*[\s\S]*?\*\//g;
     const metaRe = /==userstyle==[\s\S]*?==\/userstyle==/i;
@@ -219,5 +262,5 @@ var usercss = (function () {
     // FIXME: validate variable formats
   }
 
-  return {buildMeta, buildCode};
+  return {buildMeta, buildCode, colorParser};
 })();
diff --git a/manage.html b/manage.html
index 9f7752ca..308c954f 100644
--- a/manage.html
+++ b/manage.html
@@ -139,6 +139,8 @@
   <script src="manage/filters.js"></script>
   <script src="manage/updater-ui.js"></script>
   <script src="manage/object-diff.js"></script>
+  <script src="js/usercss.js"></script>
+  <script src="manage/config-dialog.js"></script>
   <script src="manage/manage.js"></script>
 </head>
 
diff --git a/manage/config-dialog.js b/manage/config-dialog.js
new file mode 100644
index 00000000..6c7d9905
--- /dev/null
+++ b/manage/config-dialog.js
@@ -0,0 +1,121 @@
+/* global usercss messageBox */
+
+'use strict';
+
+function configDialog(style) {
+  const {colorParser} = usercss;
+  const form = buildConfigForm();
+
+  return messageBox({
+    title: `Configure ${style.name}`,
+    className: 'config-dialog',
+    contents: form.el,
+    buttons: [
+      t('confirmSave'),
+      {
+        textContent: t('confirmDefault'),
+        onclick: form.useDefault
+      },
+      t('confirmCancel')
+    ]
+  }).then(result => {
+    if (result.button !== 0 && !result.enter) {
+      return;
+    }
+    return form.getVars();
+  });
+
+  function buildConfigForm() {
+    const labels = [];
+    const vars = deepCopy(style.vars);
+    for (const key of Object.keys(vars)) {
+      const va = vars[key];
+      let appendChild;
+      if (va.type === 'color') {
+        va.inputColor = $element({tag: 'input', type: 'color'});
+        // FIXME: i18n
+        va.inputAlpha = $element({tag: 'input', type: 'range', min: 0, max: 1, title: 'Opacity', step: 'any'});
+        va.inputColor.onchange = va.inputAlpha.oninput = () => {
+          va.dirty = true;
+          const color = colorParser.parse(va.inputColor.value);
+          color.a = Number(va.inputAlpha.value);
+          va.value = colorParser.format(color);
+          va.inputColor.style.opacity = color.a;
+        };
+        appendChild = [va.label, va.inputColor, va.inputAlpha];
+      } else if (va.type === 'checkbox') {
+        va.input = $element({tag: 'input', type: 'checkbox'});
+        va.input.onchange = () => {
+          va.dirty = true;
+          va.value = String(Number(va.input.checked));
+        };
+        appendChild = [va.input, $element({tag: 'span', appendChild: va.label})];
+      } else if (va.type === 'select') {
+        va.input = $element({
+          tag: 'select',
+          appendChild: Object.keys(va.select).map(key => $element({
+            tag: 'option', value: key, appendChild: va.select[key]
+          }))
+        });
+        va.input.onchange = () => {
+          va.dirty = true;
+          va.value = va.input.value;
+        };
+        appendChild = [va.label, va.input];
+      } else {
+        va.input = $element({tag: 'input', type: 'text'});
+        va.input.oninput = () => {
+          va.dirty = true;
+          va.value = va.input.value;
+        };
+        appendChild = [va.label, va.input];
+      }
+      labels.push($element({
+        tag: 'label',
+        className: `config-${va.type}`,
+        appendChild
+      }));
+    }
+    drawValues();
+
+    function drawValues() {
+      for (const key of Object.keys(vars)) {
+        const va = vars[key];
+        const value = va.value === null || va.value === undefined ?
+          va.default : va.value;
+
+        if (va.type === 'color') {
+          const color = colorParser.parse(value);
+          va.inputAlpha.value = color.a;
+          va.inputColor.style.opacity = color.a;
+          delete color.a;
+          va.inputColor.value = colorParser.formatHex(color);
+        } else if (va.type === 'checkbox') {
+          va.input.checked = Number(value);
+        } else {
+          va.input.value = value;
+        }
+      }
+    }
+
+    function useDefault() {
+      for (const key of Object.keys(vars)) {
+        const va = vars[key];
+        va.dirty = va.value !== null && va.value !== undefined &&
+          va.value !== va.default;
+        va.value = null;
+      }
+      drawValues();
+    }
+
+    function getVars() {
+      return vars;
+    }
+
+    return {
+      el: labels,
+      useDefault,
+      getVars
+    };
+  }
+}
diff --git a/manage/manage.js b/manage/manage.js
index 9524406e..4e8d447a 100644
--- a/manage/manage.js
+++ b/manage/manage.js
@@ -2,6 +2,7 @@
 /* global filtersSelector, filterAndAppend */
 /* global checkUpdate, handleUpdateInstalled */
 /* global objectDiff */
+/* global configDialog */
 'use strict';
 
 let installed;
@@ -282,128 +283,20 @@ Object.assign(handleEvent, {
   },
 
   config(event, {styleMeta: style}) {
-    const form = buildConfigForm();
-
-    messageBox({
-      title: `Configure ${style.name}`,
-      className: 'config-dialog',
-      contents: form.el,
-      buttons: [
-        t('confirmSave'),
-        {
-          textContent: t('confirmDefault'),
-          onclick: form.useDefault
-        },
-        t('confirmCancel')
-      ]
-    }).then(result => {
-      if (result.button !== 0 && !result.enter) {
+    configDialog(style).then(vars => {
+      if (!vars) {
+        return;
+      }
+      const keys = Object.keys(vars).filter(k => vars[k].dirty);
+      if (!keys.length) {
         return;
       }
       style.reason = 'config';
-      const vars = form.getVars();
-      let dirty = false;
-      for (const key of Object.keys(vars)) {
-        if (vars[key].dirty) {
-          dirty = true;
-          style.vars[key].value = vars[key].value;
-        }
-      }
-      if (!dirty) {
-        return;
+      for (const key of keys) {
+        style.vars[key].value = vars[key].value;
       }
       saveStyleSafe(style);
     });
-
-    function buildConfigForm() {
-      const labels = [];
-      const vars = deepCopy(style.vars);
-      for (const key of Object.keys(vars)) {
-        const va = vars[key];
-        let appendChild;
-        if (va.type === 'color') {
-          va.inputColor = $element({tag: 'input', type: 'color'});
-          // FIXME: i18n
-          va.inputAlpha = $element({tag: 'input', type: 'range', min: 0, max: 255, title: 'Opacity'});
-          va.inputColor.onchange = va.inputAlpha.oninput = () => {
-            va.dirty = true;
-            va.value = va.inputColor.value + Number(va.inputAlpha.value).toString(16);
-            va.inputColor.style.opacity = va.inputAlpha.value / 255;
-          };
-          appendChild = [va.label, va.inputColor, va.inputAlpha];
-        } else if (va.type === 'checkbox') {
-          va.input = $element({tag: 'input', type: 'checkbox'});
-          va.input.onchange = () => {
-            va.dirty = true;
-            va.value = String(Number(va.input.checked));
-          };
-          appendChild = [va.input, $element({tag: 'span', appendChild: va.label})];
-        } else if (va.type === 'select') {
-          va.input = $element({
-            tag: 'select',
-            appendChild: Object.keys(va.select).map(key => $element({
-              tag: 'option', value: key, appendChild: va.select[key]
-            }))
-          });
-          va.input.onchange = () => {
-            va.dirty = true;
-            va.value = va.input.value;
-          };
-          appendChild = [va.label, va.input];
-        } else {
-          va.input = $element({tag: 'input', type: 'text'});
-          va.input.oninput = () => {
-            va.dirty = true;
-            va.value = va.input.value;
-          };
-          appendChild = [va.label, va.input];
-        }
-        labels.push($element({
-          tag: 'label',
-          className: `config-${va.type}`,
-          appendChild
-        }));
-      }
-      drawValues();
-
-      function drawValues() {
-        for (const key of Object.keys(vars)) {
-          const va = vars[key];
-          const value = va.value === null || va.value === undefined ?
-            va.default : va.value;
-
-          if (va.type === 'color') {
-            va.inputColor.value = value.slice(0, -2);
-            va.inputAlpha.value = parseInt(value.slice(-2), 16);
-            va.inputColor.style.opacity = va.inputAlpha.value / 255;
-          } else if (va.type === 'checkbox') {
-            va.input.checked = Number(value);
-          } else {
-            va.input.value = value;
-          }
-        }
-      }
-
-      function useDefault() {
-        for (const key of Object.keys(vars)) {
-          const va = vars[key];
-          va.dirty = va.value !== null && va.value !== undefined &&
-            va.value !== va.default;
-          va.value = null;
-        }
-        drawValues();
-      }
-
-      function getVars() {
-        return vars;
-      }
-
-      return {
-        el: labels,
-        useDefault,
-        getVars
-      };
-    }
   },
 
   entryClicked(event) {