Admincp >> Languages & Phrases >> Search in Phrases
Search for "admin_required_register" (without quotes)
you will then have to edit the phrase:
Code:
The administrator may have required you to <a href="{1}">register</a> before you can view this page.
replace with this:
Code:
The administrator may have required you to <a href="{1}"><span style="font-size:[color=Red]20px[/color];">register</span></a> before you can view this page.