Custom colors prop support

[Kolterdyx]

Implements a feature I just suggested here.

It allows to add custom colors definition for branding/theming using the ui.colors function. It can be used as follows:

from nicegui import ui

ui.colors(my_custom_color='#ff0')
ui.button('Custom color', color='my-custom-color')

ui.run()

The javascript added to make this work probably isn't optimal, but it works as intended, so that's probably what needs to be reviewed the most

[rodja]

Thanks @Kolterdyx. Great idea. @falkoschindler will look at it after coming back from his well deserved vacation.

[falkoschindler]

Thanks for the pull request, @Kolterdyx! That's a neat addition.

I just

• refactored the JavaScript for-loop a little,
• made sure to replace the custom style tag to avoid growing the DOM when calling ui.colors repeatedly,
• added custom_colors to the docstring, and
• simplified the demo a bit.

Let's merge!