I have thought about this before and am interested in how this might be approached. (Although I doubt that I will pursue it very far myself…)
a) ExportHtml could be borrowed to create an HTML doc with the scope-names added to each element as class-names. A JavaScript (or jQuery) colour-picker could then be invoked on double-click within the HTML. However, I doubt think this approach is viable, because it would not be possible to output a local file. (Although, the content for the file could be output in an iframe or textarea for copying and saving(?).)
b) A hosted web-site could be built a-la studiostyl.es. This could store some theme files, copy one of them, modify and export it. How would we/you approach the editing of the theme file? I suppose plistlib could be used to create a dictionary, modify it and write it out as a completely new file. Or would you use some other XML tool(s) to *edit *a theme file?
c) But it might also be possible to do it entirely in ST: there is a ColorPicker Package. I have an idea about this: Instead of trying to find the current scope, and colour, within the attached theme-file, just read the currrent scope, and inject this scope, and the chosen colour, directly at the beginning of the attached theme file. It would be a very specific scope (language specific) and it will be adopted first at it occurs very early in the file. What XML feature/extension could be used to directly insert a node into a file?
Andy.