Avoid use of 'px' to specify UI styles

Replace most uses of px used to specify sizes in the UI CSS with pt. Sizes specified in pixels will, in some cases of non-standard scaling / high-DPI setups, result in physical pixels rather than any sensible unit and can lead to text that is illegibly tiny. Units specified in pt do not have this problem.

Some 1px values have been retained under the assumption these are meant to mean "hairline" and that it is therefore acceptable for them to be specified in physical pixels.

Also, remove unneeded units from zero values for consistency.

Fixes #2444.