Review mapp/ui css for theming / dark mode

[dbauszus-glx]

The mapp and the ui css in particular is a mess.

Things have been added in an unsttructured manner without review and outdated rules have never been removed.

The dialog for example has a white background colour [#fff] which makes it impossible to theme this by prepending or appending css rules to the document head.

The number of colours should be reviewed. The more colours we add the more chaotic theming becomes.

The idea of theming is that each colour has a lookup colour.

Colours should be assigned as classes.

Instead of n classes having n different background colours it is much easier to use a background-colour class for a background colour which can easily be exchanged by appending theme rules.

[dbauszus-glx]

I have created a PR for the dialog example.

#1600

[dbauszus-glx]

The dark_mode plugin script and css should be added to the mapp css / core plugins during the review of colour schemes as they are affected.