[FEAT] Add colour mixins and external colour theme support

[AdamXweb]

Describe the solution you'd like

Allow colourways and firefox themes to add some flare, keeping the arrangement of safari.
This could be done in a few ways:

User defined colour overrides:

• Can add a colour mixin to main colours to slightly tweak them to the variant: var(--gnome-switch-active-background, color-mix(in srgb, currentColor 12.5%, var(--theme-primary-color)))

Mozilla colorways override

• again, to mix the colours together, but keep the safari aesthetic. Could be same as above, but instead of variable of theme colour, could be: -moz-accent-color

Script to convert theme manifest to css

• Can convert a theme's manifest into a CSS that can be imported as an override to the colours. There's probably a better way, but always an option.