Override --white, fixes 'more info' link in empty states (#148)

Author: timja

src/main/resources/io/jenkins/plugins/darktheme/theme.css

@@ -7,6 +7,7 @@
   --very-light-grey: var(--dark-theme-bg-dark);
   --light-grey: var(--dark-theme-bg-333);
   --medium-grey: var(--dark-theme-bg-333);
+  --white: var(--dark-theme-bg-333);
 
   /* Text */
   --text-color: #c0c0c0;