Can I do a "Global" font change?

My 'customer' got feedback from a writer, who told him the Arial font, which is apparently the default font either in WP, or in my selected theme 'twentytwelve', is OK for headings, but not for long entries. Our WP development is primarily a Wiki. It contains many pages with very long entries.
Here's the customer input " I wandered around through the site a little. Mucho good stuff, no serious feedback except one and it is a DOOZY (though it should be an easy fast fix):

The whole thing is hard to read because everything is in Arial or some other sans serif font and that is a no-no as big as the wiki for large blocks of text.

Sans serif is for cutlines and headlines and small sidebars: Main body copy MUST be a serif font -- like Times Roman and its ilk -- lest you blind everyone who tries to read the whole thing in Arial.

The reason: serifs (the curlycues) create letter-word continuity for your eye; sans serif letters-words turn into line patterns with no letter/word coherence if it goes on too long."

I am being directed to change the font to something more 'readable'.

I did a search of the 'Community' and did not find anything about changing the font globally - in other words, change EVERYTHING at once, not page by page using HTML.

PLEASE help!

Mary :o(