Add CSS property for the chat content background

RELEASE.md

@@ -5,8 +5,8 @@
 3. Run `make release VERSION=5.0.0`
 4. Do a `git diff` to check if things look sane.
 5. Do a quick manual test with the `dist` files (via `index.html`)
-6. `git commit -am "New release 5.0.0"`
-7. `git tag -s v5.0.1
+6. `git commit -am "Release 5.0.0"`
+7. `git tag -s v5.0.1 m "Release 5.0.0"`
 8. Run `git push && git push --tags`
 9. Update https://conversejs.org
 10. Create `5.0.0` directory for the CDN.

sass/_chatbox.scss

@@ -204,7 +204,7 @@
             color: var(--text-color);
             overflow-y: auto;
             border: 0;
-            background-color: #ffffff;
+            background-color: var(--chat-content-background-color);
             line-height: 1.3em;
 
             video {

sass/_variables.scss

@@ -57,6 +57,8 @@ $mobile_portrait_length: 480px !default;
     --chat-head-text-color: white;
     --chat-correcting-color: var(--chat-head-color-lighten-50-percent);
 
+    --chat-content-background-color: white;
+
     --chat-topic-display: block;
     --chat-info-display: block;