Use the primary color and adjust the width when it's the only option

[nickvergessen]

Before	After

Fix #68

cc @Becker-Asano

[zorn-v]

I think change "global" styles in app is not good idea.
Your button will be looks different (and may be buggy) on others version of nextcloud.

[nickvergessen]

What exactly do you mean?

[zorn-v]

You set fixed width for registration button with fixed margin also.
What happen if width of container will modified in next nextcloud versions styles ?
What if will be decided to change login button width etc.

In previous NC versions (prior to 18 AFAIR), container width was 300px AFAIR

[nickvergessen]

This change is only available for the next version which will be only available on nextcloud 20 and later. If that button changes again we will adapt again.

[zorn-v]

Taking into account how "fast" issue like this #210 is resolving, I doubt that it will be fast.
And users may endure ugly button through several NC versions )