Implement Light Theme sheet colours

[MangoFVTT]

Is your feature request related to a problem? Please describe.
Currently only colours for dark theme exist. Once #78 is merged, we will need to add in colours for light theme as well and ensure the swap happens correctly.

Describe the solution you'd like
A set of colours that work correctly when swapping to light theme.

Version
Sheets are implemented for 0.3.0

Checklist:

• I have searched the existing issues to ensure this feature has not already been requested.