I've done that with another theme, so yes it's possible.
1) Make sure both themes are in the correct "colorthemes" folder in your "user resources" directory. (You can find this folder under Options, "Show REAPER resource path in Explorer/Finder".) In Reaper, choose the theme (in the options menu) from which you want to copy the color choices for the arrange window, tcp, mcp, etc. (aka the "donor theme").
2) Go to the actions list and run the "theme development / tweaker" window.
3) Save the theme, and name it something different that you'll recognize later. Normally a Reaper theme (with no edits) will be a
themename.ReaperThemeZip file. (Where "themename" is actually whatever the theme is named.) When you edit a theme though, it creates an extra file (which refers to the elements of the themename.ReaperThemeZip file you used) called
themename.ReaperTheme (no "zip" at the end). This new file has whatever info from the theme you can edit from the theme development / tweaker window saved in it, but nothing else--no images or anything (since those are all contained in the original themename.reaperthemezip file). Both those files are required for you to have both your theme and the edits to it. The "main" (original unedited) theme file has the "zip" at the end of the filename but the 2nd file with the "tweaks" doesn't.
*Note: If you're editing a theme and saving it like this with the "theme development / tweaker" window, and you ever want to revert to the original unedited theme, you can just delete this *.reapertheme saved file while leaving the original *.reaperthemezip file alone.
4) Close Reaper. Open that *.reapertheme file you created using a text editor (notepad or wordpad seem to work fine for this). You will need to edit one part of this so you can now use it with the other "donee" theme. Look for the following in the file (about 2/3 of the way through it):
[REAPER]
ui_img=themename.ReaperThemeZip
Change the "themename" part to whatever your 2nd theme is that you want to "paste" the colors to. Save the file.
5) Open Reaper, change your theme from the options menu (look for the name of the *.reapertheme file, not the original theme name of either theme).
And now that you're done, there's a good chance you'll want to edit things further since whatever colors work well for one theme may not work well for another in some places.
With a bit of patience though you can do a fair bit of customization.
Good luck!