Add contrast-multiplier to shades of purple

docs/themes.md

@@ -98,6 +98,7 @@ theme:
 ```yaml
 theme:
   background-color: 243 33 25
+  contrast-multiplier: 1.2
   primary-color: 50 100 49
   positive-color: 98 82 71
   negative-color: 12 77 52