Remove all hardcoded pixel values from CSS

[jeremytchang]

Currently some of our CSS uses hardcoded pixel values to handle layouts. This is not best practice, harder to maintain/extend, and less responsive.

If possible, we should remove all hardcoded pixel values in css styles unless it is for absolute positioning.

Example style with hardcoded pixel values:
height: calc(100% - 30px);

This may require significant CSS changes across the border. We should try to make use of CSS grid if possible.