Update main.css

This adds a gap between page links (page and talk page) and actions in the left sidebar and also adds a right border to active links.

This is what this commit should look like (viewing the Main Page's history):
![leftsidebar](https://f.cloud.github.com/assets/5515877/1751509/b8f417e0-65ec-11e3-8a2d-fa47a95ad250.png)