Fix media spoiler design (#6507)

- 4px rounded corners on media attachments
- Better colors/contrast for CW/media spoiler on public pages
- Fix vertical alignment of "Show more" button
- Fix layout jump when unhiding standalone media

app/javascript/styles/mastodon/stream_entries.scss

@@ -146,10 +146,10 @@
 
       a.status__content__spoiler-link {
         color: $primary-text-color;
-        background: $ui-primary-color;
+        background: $ui-base-color;
 
         &:hover {
-          background: lighten($ui-primary-color, 8%);
+          background: lighten($ui-base-color, 8%);
         }
       }
     }
@@ -214,10 +214,10 @@
 
       a.status__content__spoiler-link {
         color: $primary-text-color;
-        background: $ui-primary-color;
+        background: $ui-base-color;
 
         &:hover {
-          background: lighten($ui-primary-color, 8%);
+          background: lighten($ui-base-color, 8%);
         }
       }
     }
@@ -260,16 +260,8 @@
   }
 
   .media-spoiler {
-    background: $ui-primary-color;
-    color: $white;
-    transition: all 40ms linear;
-
-    &:hover,
-    &:active,
-    &:focus {
-      background: darken($ui-primary-color, 2%);
-      color: unset;
-    }
+    background: $ui-base-color;
+    color: $ui-primary-color;
   }
 
   .pre-header {