Fix incorrect background colour for sidebar tag pills

Fixes #568.

core/ui/MoreSideBar/Tags.tid

@@ -3,7 +3,7 @@ tags: $:/tags/MoreSideBar
 caption: {{$:/language/SideBar/Tags/Caption}}
 
 \define lingo-base() $:/language/SideBar/Tags/
-<$button to="$:/TagManager"><<lingo TagManager/Caption>></$button>
+<$button to="$:/TagManager" class="btn"><<lingo TagManager/Caption>></$button>
 
 <$list filter="[tags[]!is[system]sort[title]]">
 

themes/tiddlywiki/vanilla/base.tid

@@ -257,7 +257,7 @@ a.tw-tiddlylink-external:hover {
 	color: <<colour foreground>>;
 }
 
-.tw-sidebar-lists button {
+.tw-sidebar-lists button.btn {
 	color: <<colour sidebar-button-foreground>>;
 }