Dark theme support

[NiklasRentzCAU]

Implements support for dark / other color themes for clients over the LSP and adds a property to be read by syntheses to conform to some theme-related foreground/background/highlighting colors.

Relates to kieler/KLighD#175

[NiklasRentzCAU]

Some yet-to-dos:

• [ ] let the CLI default light theme color be null to allow for an unaffected synthesis to render its default light theme colors and set own defaults
• [ ] automatically set the background to light/dark depending on the synthesis
• [ ] set the background color to some dark color for the CLI if the system suggests that, so that dark mode synthesis also appear on a dark background per default.
• [ ] make sure that a release with this update in klighd-vscode won't break existing servers that did not update to a new KLighD release yet. I.e. the newly sent messages would crash an old server currently as they are not supported. Maybe some way to send the KLighD protocol version at initialization and only use features/messages if the server version is high enough

[a-sr]

Regarding todo no. 2, I think there is also a preference in the klighd view that enforces a white background, in this case it woun't make much sense to activate/send color preferences for a dark theme, or do you mean this by no. 2?

[NiklasRentzCAU]

Regarding todo no. 2, I think there is also a preference in the klighd view that enforces a white background, in this case it woun't make much sense to activate/send color preferences for a dark theme, or do you mean this by no. 2?

Haven't thought it through yet, but I would like to make this more automatic and consistent. For this example, if a white background is enforced, then sending no color preferences could be a good idea to allow to show the synthesis-default color theme with this option and to use the theme only in the other cases.