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That IS the css file.
But that is not where you would remove subforum display. That is in the templates.

There are many, many good reasons but chief among them would be flexibility - after years and years of requests for it the CSS working group have still not put css variables in place - and easier upgrading. We find that the vast majority of users do not want to change their CSS with most being CSS illiterate. This approach allows them to make smaller tweaks to basic colours which is what most want to do without hacking a file that may, from time to time, need to be updated by us.

So we have adopted the most common and accepted approach of doing this. In an ideal world it would not be necessary.