Drop the backticks around colour values

It’s safe to do since no CSS color could be inadvertantly wikified.

themes/tiddlywiki/vanilla/colours.tid

@@ -1,8 +1,8 @@
 title: $:/themes/tiddlywiki/vanilla/colours
 type: application/x-tiddler-dictionary
 
-primary: `#5778D8`
-background: `#fff`
-foreground: `#333`
-pagebackground: `#ececec`
-medium: `#bbb`
+primary: #5778D8
+background: #ffffff
+foreground: #333333
+pagebackground: #ececec
+medium: #bbbbbb