Making Unread Emails a Custom Colour in Thunderbird

I’m trying to change my unread emails to a custom colour in Thunderbird because the dark theme I use has made it difficult to tell the bold (unread) white from the read white. I have successfully applied a code into developer tools, specifically mailWindow1.css that allows me to change the colour to anything I want. The only issue is that anytime you update or restart Thunderbird, the changes are dropped and I have to add the file in again. Is there a way to make a permanent change in developer tools? I have tried to look this up and all I find are really complicated (at least for me) ways of doing this outside of dev tools. Thanks in advance for any ideas.

Anyone out there have any ideas?