Hi,
you can use a file named custom.css, this will be used by your theme. In there, you should use the following:
html, body {
font-family: Verdana !important;
font-size: 12px !important;
[... and so on...]
}
With this you can override the definitions.
If it doesn't affect some of your text elements, I recommend using Firebug, a Firefox Plugin, with it, you can analyze the structure of the page and you can see which definitions are in charge for this text, you can copy this and override the style definition.
Just ask if you have further problems.