Add support for light and dark mode, as per colorScheme config

[gerteck]

Is your feature request related to a problem? Please describe.

For future usage, add light and dark mode support that is toggled by the config, first default to light only (light mode for websites)

[gerteck]

Also, remove and streamline the panel.theme (light/dark) option that we have now... it should use the same thing, also consider renaming the colorScheme proeprty attribute to something more conventional...

[gerteck]

• Placeholder control components need to support dark mode text.
• Tab groups need to have dark mode borders.