Add dark theme

[MarceloRobert]

Some devices have better readability with dark theme, and also some users might just prefer that theme.

Related issues

• It may be better to do refactor: chart colors #39 first, just refactor the colors to centralize it in a single place in some way
• The theme preference might not be saved across sections without cookies (Show cookie usage banner #596 ), which would be ideal but I don't think is a blocker for this change

[mentonin]

For a workaround, the dashboard looks great with Dark Reader