Safari you can't.
However with Firefox + NoSquint + ThemeFont & Size Changer you can (click for larger image)
Firefox is a highly customizable browser, the image you see in the tool bar is called a "Persona" there are also themes too.
The Persona gives the ability to create more contrast, so you can see the type easier than the black on gray of Safari.
To customize Firefox, you need to Customize Toolbar, elements on the Toolbar can be added, removed or switched around to suit one's needs.
Theme Font & Size Changer - Affects Firefox's own type size.
NoSquint - can set a global "zoom" page level for all sites, also remmebers zoom setting for all sites, place a "zoom in / out" button on Firefoxes toolbar
I appreciate your advice, but that link seems broken!
I get an odd error message, one I've never seen before-- it's says
"It appears you're not allowed to view what you requested..."
I did find this, however.