1) Enable following flags

Go to

about:config

and set to True following:

sidebar.verticalTabs
toolkit.legacyUserProfileCustomizations.stylesheets
sidebar.revamp

2) Apply css file

Download chrome folder from this repository and put it in your profile folder
To find out its location go to

Menu -> Help -> More troubleshooting information -> Profile Directory

For flatpack users, profile folder located here:

~/.var/app/org.mozilla.firefox/.mozilla/firefox/[profile name]

3) Restart Firefox

After restart of firefox new theme would be applied