Clean up things typographically

[asmeurer]

Suggestions from #60.

• [ ] Change right sidebar to match Gamma's "card" visual style?
• [ ] Change header to match Gamma's (white on green background); current header doesn't look too attractive
• [ ] Perhaps tone down those drop shadows...
• [ ] There is a lot of green on green on green, which is probably a bad idea
• [ ] Make features on homepage link to Gamma/Live examples
• [ ] Have "Screenshots" link to Live examples
• [ ] Add searchbox for Gamma
• [ ] In general, make SymPy Live, SymPy Gamma, and the website look consistant. Can we share code somehow?
• [ ] Fix the spacing some more.
• [ ] Remove all bullets. They are unnecessary.
• [ ] Get rid of the colors in the side panel.

[mvnnn]

@asmeurer i would like to work on this issue.

[asmeurer]

Go for it (no need to ask for permission).