feat(ui): dark mode

[jkowalleck]

• fixes feat(ui): dark mode #94

added functionality

• a dark color theme for the page and the plots

added capabilities

• detects dark/light preference from browser on load and on preference-change
• renders dark/light as preferred in browser
• print layout is still light

what else

• the colors were extracted in the CSS and JSON - for easier maintenance.
• the old JS style was kept as is
• no additional libraries was introduced
• the browser-specifics in CSS were kept as is

[jkowalleck]

@ewdurbin how about a dark theme? 🎉

[ewdurbin]

Hello @jkowalleck! My focus is predominately on the operations and infrastructure of pypistats.org. I may not have time to review this PR until mid-late December.