[Feature] Support the horizon light variant

[akinsho]

Hi 👋🏿,

It would be great if the light variant of this theme could also be supported, for those times when you need to share your screen, or it's too bright for a dark theme.

I know this theme is generated using a rust crate, but it doesn't seem to incorporate light variants that can be toggled using the vim.opt.background = 'light' so maybe the functionality would need to be added to the generator tool first.