I'm not entirely sure what method is being used, as Z linked me to https://wiki.step-pr.../MassEffect.css. Overriding CSS is applied Wiki wide, and is done by either overriding a specific skin, or making global changes. In our case, both will have the same effect since we only have one skin enabled.
The link that Z provided me from Tech is not valid and will not override anything.
There is the CSS extension, but that was only installed as a workaround for the STEP guide. It is not intended for the masses to modify pages, thus altering the default look as it applies to everyone. Once MediaWiki is updated and the skin updated (which is required due to changes in MW), changes will be made to better support the Guide and similar rare cases where the CSS extension was necessary, thus making it obsolete and removed.