Allow SCSS variables to be overridden (#2987)

* Allow SCSS variables to be overridden with
  `app/javascript/pack/variables*.scss`

* Allow these SCSS variables to be overriden:
  * `$base-shadow-color`
  * `$base-overlay-background`
  * `$base-border-color`
  * `$simple-background-color`
  * `$primary-text-color`
  * `$valid-value-color
  * `$error-value-color`

app/javascript/mastodon/main.js

@@ -1,3 +1,7 @@
+// allow override variables here
+require.context('../../assets/stylesheets/', false, /variables.*\.scss$/);
+
+// import default stylesheet with variables
 require('font-awesome/css/font-awesome.css');
 require('../styles/application.scss');
 
@@ -19,6 +23,8 @@ function main() {
   Rails.start();
 
   require.context('../images/', true);
+
+  // import customization styles
   require.context('../../assets/stylesheets/', false, /custom.*\.scss$/);
 
   onDomContentLoaded(() => {

app/javascript/styles/variables.scss

@@ -13,13 +13,13 @@ $classic-secondary-color: #d9e1e8;    // Pattens Blue
 $classic-highlight-color: #2b90d9;    // Summer Sky
 
 // Variables for defaults in UI
-$base-shadow-color: $black;
-$base-overlay-background: $black;
-$base-border-color: $white;
-$simple-background-color: $white;
-$primary-text-color: $white;
-$valid-value-color: $success-green;
-$error-value-color: $error-red;
+$base-shadow-color: $black !default;
+$base-overlay-background: $black !default;
+$base-border-color: $white !default;
+$simple-background-color: $white !default;
+$primary-text-color: $white !default;
+$valid-value-color: $success-green !default;
+$error-value-color: $error-red !default;
 
 // Tell UI to use selected colors
 $ui-base-color: $classic-base-color !default;             // Darkest