Gtk-Theming-Guide VS cinnamon-transparent-panels

Look here and here, you should get some ideas.

Here's how to make your own theme or tweak an existing one https://github.com/surajmandalcell/Gtk-Theming-Guide/blob/master/creating_gtk_themes.md

here is a bit of a guide into how to do that. The gtk theme will handle the appearance of all gtk apps. QT themes can also be created separately, but QT has a setting to follow the gtk theme, so gtk is probably the way to go.

Pantheon is build/based on GTK. So, I believe that this will help you. Also, you can always visit gnome-look.org, download a theme and look through its files and see how it is made. You can also build a theme on top of it or use it as a template to get you started, ya know, open source and all those thangs. lol

You might find some extensions like this one that will get you a look you like.

This. It's the add-on (Mint menu > Addons) "Transparent panels". The compositor does not support blur effects on Mint though, so it comes without them.

Apparently there's been a bug since Cinnamon Version 5.4 (the Mint 21 upgrade brought it to Cinnamon ver 5.4.9).

Then I would try to open an issue on their github page.

I made the transparent panel with this extension.

I'm by no means an expert but I can tell you what I did with my setup. If you right click a panel you can see the 'panel settings', 'applets' and 'panel edit mode' options. If you take the time to go through these there is a lot you can do with them. I enabled the 'grouped window list' applet and used 'panel edit mode' to move it into the middle (my icons on the bottom). I enabled panel transparency by downloading this extension. I downloaded my icon pack from here.

I use this extension: https://cinnamon-spices.linuxmint.com/extensions/view/81