doc: CSS tweaks

• split selectors to one per line
• remove units from 0
• use double colon pseudo selectors consistently
• use background-color instead of the background shorthand since only the color is supposed to change
• remove leading zeros from numbers
• remove redundant values from shorthands
• use lowercase hex for colors
• use numeric values for font-weight

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines