Fix bg colour for input checkbox

[fredwu]

This PR fixes the bg colour of the checked checkboxes.

Dark mode works fine:

However, light mode currently looks like this, note that the first three boxes are checked, the last one is not, but they all look the same:

With this PR:

Took me a while to realise this was caused by styling. I was going crazy thinking my auto-checking plugin was broken. 🙈

[fredwu]

cc @mplatts

[mplatts]

this has been released btw..just forgot to add it to the changelog