You can directly ask this to the platform-ui-dev mailing-list. This e4 one isn't really relevant now.
About your custom widget, I guess by Custom you mean that you're actually taking care of some parts of their drawing? If so, then I believe it's up to you in your drawing routines to read the widget style and to choose the right colors for background/foreground accordingly. One possibility if you wish really dynamic results is that instead of reading the style, you simply rely on the background color of the parent composite, so you don't have to deal with CSS and your widget would be portable in any composite and integrate well in it without tweak