Sorry, I am swamped, try looking at https://github.com/elibroftw/matte-black-theme/blob/master/Matte%20Black%20(Blue)%20Theme%20Firefox/manifest.json, specifically
{"tab_line": "rgba(40, 83, 255, 1)",
"tab_loading": "rgba(40, 83, 255, 1)"}
You're almost there.