Theme/colors are not working for `ggm` objects

[shah-in-boots]

When adding theme or colors to ggm objects, it appears to cause an issue with scales and palettes.

> ggm(egm) + theme_egm_dark()
Scale for colour is already present.
Adding another scale for colour, which will replace the existing scale.
Error in `palette()`:
! Insufficient values in manual scale. 4 needed but only 1 provided.
Run `rlang::last_trace()` to see where the error occurred.

Tasks:

• [x] Consider splitting themes into scale functions as well
• [x] Figure out how to maintain the pre-set egm colours when available

[shah-in-boots]

Related to #20

[shah-in-boots]

By forcing the scales to match the number of potential colors with theme_egm_*(), this overrides the base colours. This may not be ideal. May want to think about splitting the theming options into a separate scale_channel_colours() style function.