Base dark mode support

[dive]

Hello there,

As you know, iOS 13 introduced the dark mode support, and it "mostly" works with MobileOrg because the application uses system components. There are some problems with custom colours for fonts, and they are fixed in the PR.

The list of changes In the pull request:

• Unified colours for all cases (I allowed myself to change some existed colours a bit, pay attention, please);
• Switched to a simple implementation for RoundedLabel (there were some artefacts);
• The white theme should work as it as before (with minor changes in gray colours).

Screenshots:

Current develop	After changes

The white theme looks like this after the changes:

[webframp]

Nice work @dive

Thanks very much!