Generally what you do in a case like that is to put the script on the page in the mainFrame.
Now frames on a page are counted from 0 to however many there are, or you can use their names (if they have names) to access them as well. The frameset page is the parent of the pages in its frames.
So to access the script on the page in the mainFrame, you could put something like this on a page in the other frame:
One caveat though, all three pages (frameset, main, and bottom) must be on the same domain.
<a href="#" onclick="parent.frames.ts('body',1); return false;"> + Larger Font</a> | <a href="#" onclick="parent.frames.ts('body',-1); return false;">+ Smaller Font</a>