Don't override browser selection colours by default

Reverts some of #4590

core/palettes/Vanilla.tid

@@ -57,6 +57,8 @@ page-background: #f4f4f4
 pre-background: #f5f5f5
 pre-border: #cccccc
 primary: #5778d8
+selection-background:
+selection-foreground:
 select-tag-background:
 select-tag-foreground:
 sidebar-button-foreground: <<colour foreground>>

themes/tiddlywiki/vanilla/base.tid

@@ -232,13 +232,13 @@ kbd {
 }
 
 ::selection {
-	background-color: <<colour primary>>;
-	color: <<colour page-background>>;
+	background-color: <<colour selection-background>>;
+	color: <<colour selection-foreground>>;
 }
 
 ::-moz-selection {
-	background-color: <<colour primary>>;
-	color: <<colour page-background>>;
+	background-color: <<colour selection-background>>;
+	color: <<colour selection-foreground>>;
 }
 
 /*