Helping out Odhinn_Allfather with his campaign, I ended up cooking up what amounts to a complete color theme kit for campaigns, and I figured I'd share it in a top level thread because it's the kind of thing a number of folks might find useful.
Basically what was involved here was picking through every page, editing page, and modal dialog and targeting them for color change of background and text color. The net result is a style sheet that provides central control over the color scheme of your entire campaign. The style sheet is around 500 lines long, but it lets you change the color scheme in a unified way by doing a find and replace on a small number of actual color codes. It's completely thorough (as far as I know, anyway), and controls colors for everything in a campaign that it can. There are a few things where color is baked into an image or inline CSS, but they're few and far between.
"http://omnichron.net/external/op/optheme.css":http://omnichron.net/external/op/optheme.css (Template) Work from this. Default colors set to various shades of gray.
"http://omnichron.net/external/op/opblue.css":http://omnichron.net/external/op/opblue.css (Blue Test) Turns everything it can the same color, to see what you can't change.
Note that this respects the campaign style colors you set in your settings as much as possible, and has a color key in it to unify the stuff that's hard-coded to orange in the default design, which is meant to be replaced with the color key that matches your primary campaign color setting. For the blue test, obviously you'd want to set all four campaign colors to that same blue.
Things you can't change as far as I can tell:
* The icons on the right of the Stream section of the dashboard are baked into PNGs as black
* The dragon head logo is an inline SVG image, might be doable from CSS but I don't think so
* The vertical timeline used in adventure logs is baked into PNGs as shades of white
* A couple of fields in Settings have their colors set by inline CSS at the lowest level