Use consistent measurements in stylesheets

[trezy]

Currently there are some stylesheets using px and others using rem. We need to make sure this is consistent throughout the project. Here are some good rules to follow:

1. px should only be used to set a consistent component sizing metric, e.g. 10px on the html element to dictate the use of rems, or to set a base font size for text on the site, e.g. 14px on the body element to dictate the use of ems.
2. rems should only be used in relation to component sizes. It should never be used to dictate font sizes.