Finalise icon colours after Vuetify 3 upgrade

[MetRonnie]

In Vuetify 2, icons had a default grey colour. In Vuetify 3 it is left up to font color inheritance, leading the icons without specified colours being (near-)black.

Some review comments on #1269 expressed a desire to retain the grey colour, however having gotten used to the darker colour I prefer it and feel it helps distinguish from disabled state. Though making them dark grey as opposed to near-black would be fine IMO

[oliver-sanders]

I took a look into some of the CSS and found some issues blocking the use of the dark theme due to hardcoded values from which icon colours are derived. Should be easy to tackle both at the same time. I have a WIP branch with most hardcoded values removed and dark theme almost there.

[MetRonnie]

Can this be closed?

[oliver-sanders]

Would rather niceify but don't have time right now.

[MetRonnie]

What is trying to be addressed here? If it's dark mode we have #336. If it's finalising icons colours I don't think that's valid anymore as we have been on these icon colours for a long time.

[hjoliver]

Seems eminently closable to me, but I'll leave to you guys.