Solarized Light theme: fix warning color/toast

I really dislike the Solarized yellow color. This should be better, and
make the warning toasts (used for locked topics, etc.) more readable.

tildes/scss/themes/_solarized.scss

@@ -77,6 +77,7 @@ $solarized-light: (
   "foreground-highlight": $fg-darkest,
   "foreground-primary": $fg-dark,
   "foreground-secondary": $fg-lightest,
+  "warning": $orange,
 );
 
 body.theme-light {

tildes/scss/themes/_theme_base.scss

@@ -479,7 +479,12 @@
     .toast-warning {
       background-color: rgba(map-get($theme, "warning"), 0.9);
       border-color: map-get($theme, "warning");
-      color: map-get($theme, "foreground-extreme");
+
+      @if (perceived-brightness(map-get($theme, "warning")) > 50) {
+        color: #000;
+      } @else {
+        color: #fff;
+      }
     }
   }