Separate variable for control box text color

This will allow setting different colors in themes where the controlbox
is a different background color from the chat view and other parts, such
as the "concord" theme.

sass/_controlbox.scss

@@ -68,6 +68,8 @@
     #controlbox {
         order: -1;
 
+        color: var(--controlbox-text-color);
+
         .open-rooms-toggle, .open-rooms-toggle .fa {
             color: var(--groupchats-header-color) !important;
             &:hover {
@@ -152,7 +154,7 @@
                 color: gray;
                 font-size: 85%;
                 &:hover {
-                    color: var(--text-color);
+                    color: var(--controlbox-text-color);
                 }
             }
         }
@@ -420,7 +422,7 @@
             min-height: 0;
 
             .brand-heading {
-                color: var(--text-color);
+                color: var(--controlbox-text-color);
                 font-size: 2em;
             }
             .chatbox-btn {

sass/_variables.scss

@@ -29,6 +29,7 @@ $mobile_portrait_length: 480px !default;
     --inverse-link-color: white;
     --text-shadow-color: #FAFAFA;
     --text-color: #666;
+    --controlbox-text-color: #666;
     --text-color-lighten-15-percent: #8c8c8c; // lighten(#666, 15%)
     --message-text-color: #555;
     --message-receipt-color: var(--green);