[WIP] [CSS Cleanup] Restructure/Cleanup CSS

Note: These are potentially volatile changes, and will need extensive testing /w discussion on how best to handle them.

Outside of just cleaning up the CSS, I will also be merging changes to enhance the view of the catalog, post form, ad banner, and other elements that impede mobile displays. I'm trying to correct things that I consider non-invasive at the moment, though there may be some changes that cause issues with other styles.

I'm not sure how terse we should be in organizing the selectors, and some thoughts would be appreciated.