Move to @friendsofreactjs/react-css-themr

[markusguenther]

Description

The currently used react-css-themr is not maintained for 2 years now. We get warnings because of the react compatibility. See issue #877

So we should move to the maintained for @friendsofreactjs/react-css-themr
https://github.com/FriendsOfReactJS/react-css-themr

Expected behavior

Get rid of the warnings

Actual behavior

Warnings from yarn

Affected Versions

Neos: 3.x

UI: 1.x

You can get this information by running composer show or using the package management module within Neos.
The UI version can also be found inside the left sidebar after version 1.0.3.

[dimaip]

In addition to this, would be nice to figure out how we are actually going to do theming, so far it seems nobody really give it a try.

[markusguenther]

Is it planed to have a bright theme and the dark one? Have seen an issue somewhere. And of course it would be cool to have backend themes :)

Also the customers will love that. Can see branded neos backends.