feat(buttons): dark-surface button variants with WCAG-AA contrast

styles/base/colors.css

@@ -53,6 +53,7 @@
 
   --color-primary-fixed:        #dce1ff;
   --color-primary-fixed-dim:    #b6c4ff;
+  --color-on-primary:           #05297a;
   --color-on-primary-fixed:     #00164e;
   --color-on-primary-fixed-variant: #264191;