While you are not allowed to alter the Adsense font face, color or size of regular ad units, the Adsense policy book has relaxed the rules in the case of Adsense Text Links which include software like Firefox, Google Pack and the new referral products.]

By default, the Adsense Text hyperlinks will inherit the layout style of your webpage but you are allowed to overwrite the default settings to make your ad links appear more prominent (which may result in better click-through rate and overall conversions).

Adsense allows publishers to alter the font face and size (not color) of the text link by enclosing the Adsense code inside the <font> HTML tag as shown below:

<font size="n" face="font1, font2..' ...">ADSENSE JAVASCRIPT CODE</font>
[-7 <= n =< +7, fonts could be Arial, Verdana, sans-serif, Courier, Helvetica, Times New Roman, Impact, etc.]

Agree, the FONT element is deprecated but that's the only option recommended by Adsense - it's not clear if the CSS Font Styles enclosed in a DIV tag can be used legally around the Google Text Links.

Word of caution: Google also says that the font size changes must be reasonably necessary to match the theme of the site. Unreasonable or abusive changes will constitute a violation of our program policies Don't misuse the freedom.