How investigate codes of linux mint themes?

[Dan-cer]

Are there kind of developer tools for desktop windows available like the dev tools in browsers?

I want to do this:
Click on the window titlebar of any linux mint program to search for the right .css file where I can edit the colors.

I installed Oomox GUI editor. However, I have difficulties to find the right item and .css, because there are more than 1 css file within my theme (Cinnamox-Kashmir-Blue).

[Termy]

I highly doubt there's any such feature. AFAIK, windows just don't ordinarily work that way, unless you may have a special window manager which can share such information a program you describe could access. If you're just looking to adjust theme files (IE: '*.css'), then you should be easily able to see which theme you're using (I assume you selected it in the 'control panel'-type thing), then edit that accordingly. Sorry if I missed something.

I have since eaten most of those words, as a result of xenopeek's. Debugging hadn't occurred to me at all!

[xenopeek]

For GTK apps you can enable the built-in debugger aka inspector: https://blog.gtk.org/2017/04/05/the-gtk-inspector/. After enabling it you can press Ctrl+Shift+i in any GTK window to open the inspector. It has a CSS tab where you can insert CSS rules in the running window. Could be useful for testing things out. But it does not link you to which specific css file is being used for that window.

For Cinnamox-Kashmir-Blue I think the cinnamon directory is the Cinnamon desktop theme. The gtk-3.0 directory is probably what you're after. I have no idea about the gtk-3.20 directory but I'm not a theme person—I just stick with the defaults.

[Dan-cer]

Thank you for your answer. The built-in debugger could be useful some day. Here, your hint at gtk-3.0 directory let me find some interesting code.
Meanwhile I got an answer from the author of the theme. He gave me another hint. However, I still not got to the solution. The author has to have a look into the new Cinnamon 5.4.x for a new overview.

I'm confident I'll get the solution from him in near future.