automatic color adjustments should take place based on the selected theme color

[j-ed]

As requested by @juliushaertl (nextcloud/server#7441 (comment)) I've created this issue.

Expected behavior

Depending on the selected theme color the notification alert color, and maybe others, should automatically be adjusted to be still visible.

Current behavior

No automatic color adjustments seems to take place at all.

Steps to reproduce

1. Open the admin settings.

2. Change the theming color to FF0000.
   

3. Check the notification symbol in the menu bar
   

4. The notification alert is nearly invisible.

Better visibility could look like this:

Environment

Server Configuration

OS: Linux 3.16.50
Web server: Apache2 2.4.29
Database: MariaDB 10.0.32
PHP version: 5.6.29
Nextcloud version: 12.0.4

Client Configuration

Browser: Mozilla Firefox 57.0.2
Operating system: Windows 7

[nickvergessen]

Hmm we are not replacing anything on the fly, but I guess we can just add a 3rd version of the bell: white+red, black+red, white+nextcloud-blue

Just need to decide how we determinate that a color is too red.

[j-ed]

Although I'm not a developer I stumbled over similar problems years ago where someone has erroneously set e.g. the character color and the background color of an application to the same color 😄
Wouldn't make sense to implement a kind of automatic check as it is described here:
https://css-tricks.com/reverse-text-color-mix-blend-mode/

[juliusknorr]

Do we actually need the red dot there, since we only show the icon if there are notifications available? @nextcloud/designers

[MorrisJobke]

Do we actually need the red dot there, since we only show the icon if there are notifications available? @nextcloud/designers

The plan was to show it permanently: #2

[nickvergessen]

We could then still do this with transparency, but I like the additional color, makes it a bit more 🎉

[MariusBluem]

The plan was to show it permanently: #2

PR for showing it permanently is in: #107

[jancborchardt]

Ok, so default to the lil red dot, and if the theming color is on the red spectrum, make the dot blue?

Can we do that with theming check and CSS/SVG magic @juliushaertl?

[juliusknorr]

Should be fairly simple, if we migrate the notifications app to SCSS.