settings: Apply dark theme to upgrade tips.

Fixes #20431 by changing dark theme colors of upgrade tips to match
colors of other dark theme tips (with the same structure as the
existing .tip class).

static/styles/dark_theme.css

@@ -875,6 +875,7 @@ body.dark-theme {
         box-shadow: 0 0 0 1px hsla(0, 0%, 0%, 0.4);
     }
 
+    .upgrade-tip,
     .tip {
         color: inherit;
         background-color: hsla(46, 28%, 38%, 0.27);