Ensure alt is readable in light and dark themes

src/routes/_components/status/Media.html

@@ -118,7 +118,8 @@
     line-height: 1;
     border: none;
     border-radius: 2px;
-    background: var(--mask-opaque-bg);
+    background: var(--alt-button-bg);
+    color: var(--alt-button-color);
   }
 
   /* Increase touch area to make it easy not to miss the button */

src/scss/themes/_base.scss

@@ -77,6 +77,9 @@
   --mask-opaque-bg: #{rgba($toast-bg, 0.8)};
   --loading-bg: #{#ededed};
 
+  --alt-button-bg: #{rgba($toast-bg, 0.8)};
+  --alt-button-color: #{$secondary-text-color};
+
   --account-profile-bg-backdrop-filter: #{rgba($main-bg-color, 0.85)};
   --account-profile-bg: #{rgba($main-bg-color, 0.9)};