css: generalize anchor :target highlighting.

Generalize :target css so that it works on other things besides forum
posts (e.g. wiki pages, /post_versions#post-version-123).

app/javascript/src/styles/base/020_base.scss

@@ -89,6 +89,11 @@ input[type="button"], input[type="submit"], button {
   padding-left: 0.5em;
 }
 
+/* Highlight link anchor targets (e.g. /forum_topics/123#forum_post_456). Use !important to override even table row backgrounds. */
+:target {
+  background: var(--target-background) !important;
+}
+
 menu {
   margin: 0;
   padding: 0;

app/javascript/src/styles/base/040_colors.css

@@ -10,6 +10,7 @@
   --error-background-color: hsl(0, 100%, 95%); /* light red */
   --success-background-color: hsl(120, 100%, 95%); /* light green */
   --warning-background-color: hsl(50, 100%, 93%); /* light yellow */
+  --target-background: #FFC;
 
   --subnav-menu-background-color: #F5F5FF;
 
@@ -65,7 +66,6 @@
   --forum-vote-up-color: green;
   --forum-vote-meh-color: goldrenrod;
   --forum-vote-down-color: red;
-  --forum-post-highlight-background-color: #FFC;
 
   --comment-sticky-background-color: var(--subnav-menu-background-color);
 
@@ -335,7 +335,6 @@ body[data-user-theme="dark"] {
   --forum-vote-up-color: var(--green-1);
   --forum-vote-meh-color: var(--yellow-1);
   --forum-vote-down-color: var(--red-1);
-  --forum-post-highlight-background-color: var(--blue-0);
 
   --jquery-ui-widget-content-text-color: var(--text-color);
   --jquery-ui-widget-content-background: var(--grey-2);
@@ -419,6 +418,8 @@ body[data-user-theme="dark"] {
   --selected-related-tag-background-color: var(--link-color);
   --selected-related-tag-color: white;
 
+  --target-background: var(--blue-0);
+
   --tag-count-indicator-frown-color: var(--red-1);
   --tag-count-indicator-meh-color: var(--yellow-1);
   --tag-count-indicator-smile-color: var(--green-1);

app/javascript/src/styles/specific/forum.scss

@@ -12,10 +12,6 @@ div.list-of-forum-posts {
       font-weight: bold;
     }
 
-    &:target {
-      background-color: var(--forum-post-highlight-background-color);
-    }
-
     .vote-score-up {
       color: var(--forum-vote-up-color);
     }