Install

• Download the firefox theme
• Firefox search bar -> type about:config -> accept
• about:config search bar -> type toolkit.legacyUserProfileCustomizations.stylesheets -> set to true
• Terminal ->

name=a-firefox-theme
git clone https://github.com/Skalyaeve/$name.git
file=userChrome.css
dir=$(find ~/.mozilla -maxdepth 1 -name "*firefox*")
find $dir -type d -name *.default* | xargs -I {} mkdir -p {}/chrome
find $dir -type d -name *.default* | xargs -I {} mv {}/chrome/$file {}/chrome/$file.bak 2>/dev/null
find $dir -type d -name *.default* | xargs -I {} cp $name/src/$file {}/chrome
rm -rf $name

Font here