[next] fix(REUSE): add SPDX header to a new file

[backportbot]

Backport of PR #5709

[susnux]

Not needed as already fixed here. But 🤷

Also it will be overwritten later by any update

[ShGKme]

Also it will be overwritten later by any update

Not for dark.css

[susnux]

Not for dark.css

Well depends how we want to update it^^
I am thinking about spinning up a nextcloud server (like for cypress) and generate the styles from there...

[ShGKme]

Not for dark.css

Well depends how we want to update it^^
I am thinking about spinning up a nextcloud server (like for cypress) and generate the styles from there...

I also need it for Talk Desktop to have global styles and themes for different server versions. So, I'm already working on such a tool in a docker image (almost done, needs some optimizations and reuse parsing).