Move .btn-used changes into .topic-voting

These were inadvertently also affecting the buttons on the /groups page,
so this moves them to specifically apply only to the topic voting
button.

tildes/scss/modules/_btn.scss

@@ -33,13 +33,12 @@
 }
 
 .btn-used {
-  background-color: darken($violet, 3%);
-  border-color: darken($violet, 8%);
-  color: #fff;
+  border-color: darken($violet, 3%);
+  color: $violet;
 
   &:hover {
-    background-color: darken($blue, 3%);
-    border-color: darken($blue, 8%);
+    background-color: darken($violet, 3%);
+    border-color: darken($violet, 8%);
     color: #fff;
   }
 }

tildes/scss/modules/_topic.scss

@@ -139,7 +139,15 @@
   }
 
   &.btn-used {
+    background-color: darken($violet, 3%);
+    border-color: darken($violet, 8%);
     border-style: solid;
+    color: #fff;
+
+    &:hover {
+      background-color: darken($blue, 3%);
+      border-color: darken($blue, 8%);
+    }
   }
 }