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Each theme has it's own CSS file. So you could allow users to choose a theme for which you have adjusted the CSS to make the type larger.
However, most browsers will allow type to be enlarged through a simple menu choice. This generally works whether the the font is specified in pixels or ems.
I can enlarge the type here on xoops.org to +300% using an old version of IE 5 for the Mac simply by selecting View/Text Zoom from the menu.
Doesn't get much easier than that!.
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