Well first of all (maybe it is offencive > not meant)why still on v1? V2.x.x is much saver and works with most common practices on the web theseday, where v1 has a lot of old code, which make it hard to use, and likely you should already notice errors using modern browsers.!! So i would suggest upgrade.
Your question :
Graphite, assuming Graphite v2 theme? by dj senki (Hutter Mihály).
Go with a good editor for css into the theme folder for Graphite and find : style css (backup always before making changes):
you will find 18 times troughout font-size: xxxx.px starting with the overall body size of 12px
NO w if you are only concerned with forum starting around line 268 forum has special settings..afaics X3
But remember allover 18 times... so maybe you have to change them too.
Please take the advice and try to upgrade..
Have a nice day and success