Are you aware that your Windows OS can control the Titlebar ?

Computer Settings > Personalisation > Colours If I set up 'choose your colour' and select 'Custom' and set both default windows mode and app mode to 'Light' AND set up a default accent colour as eg: 'seafoam teal' AND for 'Show accent colour on the following surfaces' do not select 'Titlebars and window borders.

In Thunderbird if using System Theme or Light theme Then Titlebar will have a black font on pale white background by default.

If in Computer Settings > Personalisation > Colours 'Show accent colour on the following surfaces' - Select 'Titlebars and window borders.'

In Thunderbird the titlebar will now have a 'seafoam teal' background and a white font (computer auto choose white or black font based on the background 'accent' colour).

Computer Settings > Personalisation > Colours If I set up 'choose your colour' and select 'Custom' and set both default windows mode and app mode to 'Dark' I also set Thunderbird Themes as 'Dark' The Titlebar instructions have not changed so I'm still seeing Seafoam Teal background and white font.

The point of me telling you all this, is really just to explain how your Windows 10 computer can now control your Titlebar if you have those settings enabled. Although sometimes you do need to restart Thunderbird if you alter a theme to see results.

It would help if you could supply a lot more information about your set up: What settings have you got set up in this location on computer: Computer Settings > Personalisation > Colours

In Thunderbird, what Theme have you selected? Light ,Dark or System theme ?

Regarding the 'userChrome.css' file... Have you been using the 'userChrome.css' before or is this your first attempt ? Does it work for other html css code effecting other aspects of Thunderbird OR does nothing work ? It would help if you could upload some images that show: you have the 'chrome' folder in the 'profile name' folder and the 'userchrome.css' file in the 'chrome' folder and that it's Type is a CSS file and not a Text document.

Then open the 'userChrome.css' file and post an image showing the code you currently using - I know it's not working but what is not working ?

Solved. Thunderbird does not follow Windows settings. Must set in userChrome

/* Change the color of the title bar */

1. titlebar {

   background-color: #008A8A !important; /* Seafoam teal */
   color: white 

}