Rework theming logic to invert header icon color

[justin-sleep]

The current method of determining whether or not to invert the color for header icons (after 68a63ad) is insufficient. Currently, it allows darker icons on a fairly dark background, e.g. my current theme color:

One possible solution would be to raise the lightness threshold for inverting the colors. This is what it looks like after I raised the threshold past 55 (my background color's lightness):

As what looks 'better' may vary from person to person, another possible solution to this would be to simply allow the user a button in the theming interface to toggle dark/light header icons.

Steps to reproduce

1. Open theming settings
2. Change background color to a value straddling the lightness threshold (e.g. #745CBA)

Expected behaviour

The header icon colors should not be dark.

Actual behaviour

The header icon colors invert to their dark variants.

[MariusBluem]

@juliushaertl

[juliusknorr]

@justin-sleep Makes sense, but we should make sure that there isn't an issue with another color with this new threshold.

[nickvergessen]

@juliushaertl could you have a look again.

Seems like OCA.Theming.inverted is set to true but not used by the header menu:

Notifications icon is black because it uses the value.
https://github.com/nextcloud/notifications/blob/ca1977cf6ee28c835985892ff6f74c6f13630db8/js/app.js#L388

This worked in 11, now broken on 12

[MorrisJobke]

Fixed in #6344

[jancborchardt]

Good proposal @justin-sleep! More design feedback and contributions are appreciated :)