Cannot control syntax highlighting's tab width

Tabulation character (0x09, \t) is always rendered with a width of 8 spaces.

Our projects uses 4 spaces for the width of tabs, which makes them difficult to look at from Gitlab.

Not sure if this is Firefox's default or if the highlighter sets this (Chromium does the same). Couldn't find a configuration for Gitlab (gitlab.rb, GUI or even CSS from 'rouge' library) nor for Firefox.