Use userstyle.css
body {
font-size: 12px;
/* sets body text font size in pixels */
color: #000000;
/* sets body text font colour */
}
or any other css like font-family, font-style, font-weight etc.
The above just changes the body text so you may want to change other elements such as
table th, table td {
/* insert css here */
}
blockquote {
/* insert css here */
}
code {
/* insert css here */
}
Some of the posts on the Share Your CSS thread may help.