Remove all id selectors from CSS

Using id selectors (`#foo`) in CSS makes dealing with selector specificity a nightmare, since any selector with an id selector in it will always override a selector without one. We can always add a class to an element and use the class in the selector instead.

Note that this will become irrelevant if we end up using CSS Modules/styled-components/CSS-in-JS.

Search the repository's CSS for ID selectors and identify every affected rule and element. Replace the selectors with class-based equivalents, then verify that the affected styling remains correct; the issue provides no specific files or tests.

Written by the indexing model from the issue text.