edit how "older posts" looks

Determine if “Older Posts” has an id or class assigned to it. In Firefox you can right-click on the phrase and choose Inspect Element, which will tell you the name of the id (#) or class (.).

Once you know the id/class, go to the Stylesheet under Appearance -> Editor. Search for the id/class, then modify the style to increase the font-size and add font-weight: bold;