The best way to figure out what you need too hook into for CSS is to use the Chrome inspection tool. Every other browser should also have the exact same functionality. This article from Google does a great job of showing you how to go about it.
So for the way I found .raidProgressWidgetBossNameMultiGroup was by right clicking on the widget, click inspect, then hover around the html in the console too see exactly which element I want too edit. You can, right in the console, tinker with values to see exactly what you want, then write the CSS into your site CSS section. I know that's fairly advanced, but it would be extremely difficult to build a UI that replicates this any better than what's already built into Chrome and other browsers.
I've been fiddling a little with another site (not going to jump ship .. but I like to tinker occasionally and they wanted me to beta test and give some feedback .. suspect they regretted it :P ) ... They don't have anywhere as near as many options as you guys as far as actual pages/content etc is concerned, for a start off, however, might be worth checking out for ideas?
This is their interface for page amendments:
It doesn't all work at the moment (beta!) .. but the basics are there .. unfortunately some of their basics are anything but .. I think it's always a bit of a balance between what you can and do provide, and what you should .. V4 was possibly a bit too far one way, and V5 is a bit too far the other ... what you need is V4 with the V5 look :P
The image for the raid is queued up ready to go with the next update. I may be able to get it out today, but most likely tomorrow. The raid and bosses was just database work making it easy to get out instantly.
This is showing now .. it's very shiny! Thank you :-)