fix(color): Theme edit preview not working with LVGL styles

[philmoz]

After the libopenui refactor the theme editor preview no longer worked with screen elements defined entirely with LVGL styles (e.g. toggle switch and text button).

This PR cleans up the style management and fixes the theme preview to work correctly.

[raphaelcoeffic]

@philmoz may I get a freePreviewStyle() to be called on page exit? (obviously after all LVGL components on the preview have been destroyed)

[philmoz]

may I get a freePreviewStyle() to be called on page exit? (obviously after all LVGL components on the preview have been destroyed)

Not sure about this. It could result in memory fragmentation if a user is playing with themes (free does not appear to coalesce adjacent blocks).

It's not using a lot of memory and will be gone on next restart.

[philmoz]

I've reduced the RAM overhead of the LVGL styles by converting them to constant styles where possible.