Restyle and relayout the back/forward arrows

[evinism]

Right now back/forward arrows are a little non-obvious. I want to make them bigger and more obvious. Having them take up the entire width of the aside makes sense to me. In rudimentary diagram:

------------|
header      |
------------|
| problem   |
| text      |
|           |
|           |
|           |
|-----------|
|  <  |  >  |
------------|
footer      |
------------|