Hmmm.
The short answer is /osta/css/themes/custom.css
Although I find it odd that you are using a custom color scheme successfully, but all of the colors show up as #000000 in Theme Options.
If /osta/css/themes/custom.css doesn’t have your custom colors, you can use a tool such as PicPick to find the values for the colors in your existing color scheme.
The good news is that all of the Theme Options have been moved into the database… future upgrades will preserve settings.