Built-in themes support

[blake-mealey]

Is there any appetite for a PR which adds better theming support?

My thinking is the project could ship with at least 2 themes (light and dark), and could provide a new command to choose the active theme. Users could still override this with custom CSS in .vscode

A nice-to-have would be to detect if the user's current VSCode theme is a light or dark theme and then default to the appropriate board theme.