add color gradient support to border/focus ring

[cptn-cosmo]

please add an option to allow for color gradients in the border and focus ring configuration, like hyprland does in the default config

[YaLTeR]

Haven't looked at what Hyprland does but should be doable I think.

[jgregoire]

That would look incredibly slick.

[YaLTeR]

I added gradient borders in 48f0f6f.

[jgregoire]

<3