First of all there are a couple of errors in your HTML markup.
1. a tags are inline elements and nesting block elements such as div and ul in them are not allowed
2. You shouldn't have text directly in a ul tag, for each item in the tag insert an li tag and have text in it like this
To answer you question, switch the order of the tags in your selector so that it selects the div inside a tags.
/* insert your style definitions here */
Als you probably would want to give the a tag a class of some kind so that you are not selecting all a tags within your webpages.