Can I Edit a Google Chrome Theme?

A theme is a special kind of extension that changes the way the browser looks. Themes are packaged like regular extensions, but they don't contain JavaScript or HTML code. You can download any theme and modify it by editing the manifest.json file.

Colors are in RGB format. To find the strings you can use within the "colors" field, look for kColor* strings in theme_service.cc. Image resources use paths relative to the root of the extension. You can override any of the images that are specified by kThemeableImages in theme_service.cc. Just remove the "IDR_" and convert the remaining characters to lowercase.

Reference: https://developer.chrome.com/extensions/themes

As was mentioned, you can find it in appdata folder, e.g.: C:\Users\mizer\AppData\Local\Google\Chrome\User Data\Default\Extensions\

Hint: Sort folders by date and you can easily find the one you want :)

For Chromium, this doesn't seem to work, as the manifest.json is located in: /home/[[USER]]/.config/chromium/Default/Extensions/cdfdkmklcjlnnnlnplffpdiekfhkpbme/2.1_0

And modifying and saving this, doesn't reflect on a change in Chromium. I presume Chromium (nearly the same as Chrome), loads up this config file in cache of some sorts...

You can find it in your appdata folders. Mine was located at:

C:\Users\home\AppData\Local\Google\Chrome\User    
Data\Default\Extensions\mcphcjcjgkjmbphkfjleamgkinaeebnm\1.1_0

You might just want to make a search query in your appdata for "manifest.json", and open a few until you find it.