[ADD] runbot_merge: color theme switcher

Uses localStorage rather than the odoo backend cookie because
Odoo *requires* the cookie, it can't fallback on the system theme, and
we might want a different color theme on the frontend and backend
since the frontend dark theme is pretty half-assed.

Also load the file by hand, as adding it to `assets_frontend` causes a
flash of system color theme. It would probably be possible to create
an asset which is not deferred or lazy (but wouldn't be async either
which is a shame), but I can't be arsed.

Part of #1088

runbot_merge/static/scss/revariable.scss

@@ -1,6 +1,6 @@
 // resets a bunch of CSS rules to either `inherit` color or use CSS variables,
 // such that they properly follow CSS variables
-:root {
+:root, :root.light {
   color-scheme: light dark;
 
   // dunno why it's not global
@@ -17,11 +17,11 @@
   // adjusting the conversion just set the value we want
   --danger-bg-rgb: 242, 222, 222;
 }
+
 @function invlight($value) {
   @return hsl(hue($value), saturation($value), 100% - lightness($value));
 }
-@media (prefers-color-scheme: dark) {
+@mixin darkrules {
-  :root {
     $ibc: invlight($body-color);
     --body-color: #{$ibc};
     --body-color-rgb: #{to-rgb($ibc)};
@@ -41,8 +41,17 @@
       ))};
     }
     --danger-bg-rgb: 41, 10, 10;
+}
+
+:root.dark {
+  @include darkrules;
+}
+@media (prefers-color-scheme: dark) {
+  :root:not(.light) {
+    @include darkrules;
   }
 }
+
 body {
   font-family: inherit;
 }

runbot_merge/static/src/js/runbot_merge.js

@@ -0,0 +1,42 @@
+function setColorScheme(t) {
+    const classes = document.documentElement.classList;
+    classes.remove('light', 'dark');
+
+    const buttons = document.querySelectorAll('.theme-toggle button');
+    for(const button of buttons) {
+        button.classList.toggle(
+            'active',
+            (t === 'light' && button.classList.contains('fa-sun-o'))
+            || (t === 'dark' && button.classList.contains('fa-moon-o'))
+            || (t !== 'light' && t !== 'dark' && button.classList.contains('fa-ban'))
+        );
+    }
+
+    switch (t) {
+    case 'light': case 'dark':
+        classes.add(t);
+        window.localStorage.setItem('color-scheme', t);
+        break;
+    default:
+        window.localStorage.removeItem('color-scheme');
+    }
+}
+
+window.addEventListener("click", (e) => {
+    const target = e.target;
+    if (target.matches(".btn-group.theme-toggle button")) {
+        setColorScheme(
+            target.classList.contains('fa-sun-o') ? 'light' :
+                target.classList.contains('fa-moon-o') ? 'dark' :
+                    null
+        );
+    }
+});
+
+if (document.readyState === "loading") {
+    document.addEventListener("DOMContentLoaded", (e) => {
+        setColorScheme(window.localStorage.getItem('color-scheme'));
+    });
+} else {
+    setColorScheme(window.localStorage.getItem('color-scheme'));
+}

runbot_merge/views/templates.xml

@@ -640,4 +640,20 @@ action = batches
             </tbody>
         </table>
     </template>
+    <template id="theme-script" inherit_id="website.layout">
+        <xpath expr="//head" position="inside">
+            <script type="text/javascript"
+                    src="/runbot_merge/static/src/js/runbot_merge.js"
+                    async="async"/>
+        </xpath>
+    </template>
+    <template id="theme-toggle" inherit_id="website.placeholder_header_call_to_action">
+        <xpath expr="." position="inside">
+            <div class="btn-group btn-group-toggle theme-toggle">
+                <button class="btn btn-outline-secondary fa fa-sun-o"/>
+                <button class="btn btn-outline-secondary fa fa-moon-o"/>
+                <button class="btn btn-outline-secondary fa fa-ban active"/>
+            </div>
+        </xpath>
+    </template>
 </odoo>