Fix, cleanup, and simplify CSS

[parlough]

• Use same search icon on both top nav and sidenav search bars (rather than pulling one from a website)
• Use a local SVG (from Material Symbols) for search icon
• Remove now unused search PNG
• Remove unnecessary padding causing horizontal scrollbar on offcanvas sidenav menu
• Remove various unused and debugging styles
• Adjust some formatting for easier reading of theme variables
• Remove unnecessary pixel specifiers
• Use shorthand margin and padding syntax where possible

[srawlins]

Thanks for running through all of these!