Customise colors for work, break and long break

[sekwah41]

At some point, I'd like to explore adding support for the Luxafor Pomodoro timer, though for now for both choosing what colours work best for each person as well as preparing for integration with other services or devices we should add colour selectors for all three of the modes.

[AugustasV]

Hi @sekwah41 ! Feel free to contribute!