[Theme:pinboard] Bold in sidebar's menu

Hi everyone,
in my website I made two menus with different pages and I put one in the “Sidebar Top” and one in the “Sidebar Right” to have just some of the pages in the top sidebar and some of the rest in the lateral one.

I’d like to change the character of the lateral sidebar to bold, but i dunno how to do it, how can i do this?
Thanx