Instructions 'cause I don't know how to use this site properly :p hahaha
Bonjour! I made this simple theme using codes from other themes, spliced into a very simple one only for transparency and better view of the Firefox theme image.
The original codes came from these two themes:

[http://freestyler.ws/style/97344/gt-deepdark-for-google-legacy/gt-deepdark-for-google-legacy]
[userstyles's userstyles's KARA's Jung Nicole tumblr logo/firefox-minimal-elegance-theme-transparency]

There is no intent to injure the rights of any of them.

Here are some color codes to change the color of the address bar (urlbar), search bar and etc.
See the code #969975 in front of the background below in the blue code? It is the color of the address bar. This color is a brownish yellow that suits some clear themes.
I'll leave other colors, just replace the code, it is easy and fast. If you want just transparent bar, write transparent instead of the color code. (Do not forget the key # in front of the code, otherwise it will not work. Also do not remove the spaces.)

#066D9E almost oil blue
#850000 deep red
#995E08 brown cinnamon
#011924 real oil blue
#31056E dark purple
#00FFBA green water
#005941 dar green water
#1D7D05 sap green
#FF00B1 magenta

For more colors, visit the https://color.adobe.com/pt/create/color-wheel/, which is a paradise. Copy the desired HEX color, not RGB.
Do not forget to contrast the color of the letters of the address bar with the background. The best thing is to let black #000 or white #fff, but if you want to change, it is a choice. A line below the field where you changed the color of the address bar (urlbar), is written color: #fff (just below the opacity: 0.7 field), there just put the desired color instead of #fff.