How to customize panel?

[plush warren]

That has specific pixel width and with border radius
or where can I find the file that I can modify the panel?

[night locust]

If you know css look for the cinnamon.css file inside you theme folder. Themes are stored globally at /usr/share/themes or locally at ~/.local/share/themes or ~/themes

[plush warren]

What classname or id name of the panel? Because I tried to edit the classname panel it didn't change the default panel

[night locust]

.panel-top, .panel-bottom, .panel-left, .panel-right