DEV: Let themes extend color definitions (#10429)

Themes can now declare custom colors that get compiled in core's color definitions stylesheet, thus allowing themes to better support dark/light color schemes. 

For example, if you need your theme to use tertiary for an element in a light color scheme and quaternary in a dark scheme, you can add the following SCSS to your theme's `color_definitions.scss` file: 

```
:root {
  --mytheme-tertiary-or-quaternary: #{dark-light-choose($tertiary, $quaternary)};
}
```

And then use the `--mytheme-tertiary-or-quaternary` variable as the color property of that element. You can also use this file to add color variables that use SCSS color transformation functions (lighten, darken, saturate, etc.) without compromising your theme's compatibility with different color schemes.

app/assets/javascripts/admin/components/ace-editor.js

@@ -33,6 +33,15 @@ export default Component.extend({
     }
   },
 
+  @observes("placeholder")
+  placeholderChanged() {
+    if (this._editor) {
+      this._editor.setOptions({
+        placeholder: this.placeholder
+      });
+    }
+  },
+
   @observes("disabled")
   disabledStateChanged() {
     this.changeDisabledState();
@@ -72,7 +81,6 @@ export default Component.extend({
 
   didInsertElement() {
     this._super(...arguments);
-
     loadScript("/javascripts/ace/ace.js").then(() => {
       window.ace.require(["ace/ace"], loadedAce => {
         loadedAce.config.set("loadWorkerFromBlob", false);
@@ -85,7 +93,7 @@ export default Component.extend({
 
         editor.setTheme("ace/theme/chrome");
         editor.setShowPrintMargin(false);
-        editor.setOptions({ fontSize: "14px" });
+        editor.setOptions({ fontSize: "14px", placeholder: this.placeholder });
         editor.getSession().setMode("ace/mode/" + this.mode);
         editor.on("change", () => {
           this._skipContentChangeEvent = true;

app/assets/javascripts/admin/components/admin-theme-editor.js

@@ -1,3 +1,4 @@
+import I18n from "I18n";
 import { next } from "@ember/runloop";
 import Component from "@ember/component";
 import discourseComputed from "discourse-common/utils/decorators";
@@ -30,9 +31,17 @@ export default Component.extend({
   activeSectionMode(targetName, fieldName) {
     if (["settings", "translations"].includes(targetName)) return "yaml";
     if (["extra_scss"].includes(targetName)) return "scss";
+    if (["color_definitions"].includes(fieldName)) return "scss";
     return fieldName && fieldName.indexOf("scss") > -1 ? "scss" : "html";
   },
 
+  @discourseComputed("currentTargetName", "fieldName")
+  placeholder(targetName, fieldName) {
+    return fieldName && fieldName === "color_definitions"
+      ? I18n.t("admin.customize.theme.color_definitions.placeholder")
+      : "";
+  },
+
   @discourseComputed("fieldName", "currentTargetName", "theme")
   activeSection: {
     get(fieldName, target, model) {

app/assets/javascripts/admin/models/theme.js

@@ -72,7 +72,7 @@ const Theme = RestModel.extend({
     }
 
     return {
-      common: [...common, "embedded_scss"],
+      common: [...common, "embedded_scss", "color_definitions"],
       desktop: common,
       mobile: common,
       settings: ["yaml"],

app/assets/javascripts/admin/templates/components/admin-theme-editor.hbs

@@ -87,4 +87,4 @@
   <pre class="field-error">{{error}}</pre>
 {{/if}}
 
-{{ace-editor content=activeSection editorId=editorId mode=activeSectionMode autofocus="true"}}
+{{ace-editor content=activeSection editorId=editorId mode=activeSectionMode autofocus="true" placeholder=placeholder}}