Cannot save Email in Builder

Just did a fresh install of Mautic. Created an email and change the color of a button in the Builder. Whe ni try to save the changes with 'Close builder ’ the spinner keeps spinning. In the developer console i see ‘libraries.js?v6e18ae33:571 Uncaught InvalidCharacterError: Failed to execute ‘setAttribute’ on ‘Element’: ‘!important;’ is not a valid attribute name.’

This seems to be only an issue of the Oxygen theme. Fixed here: https://github.com/mautic/mautic/pull/2474

This is great! I had exactly the same problem with a hosted Mautic account. The solution mentioned on github works for the hosted version as well. The steps I took that worked for me are:
1: download the Oxygen them from your mautic setup (click the cogwheel right top and then themes)
2: unpack zipfile
3: find these 2 files and copy content from github over to your own files with a text editor
https://github.com/escopecz/mautic/blob/ed6a9bc33cd01217c6609f4d406493cc8f0a97dd/themes/oxygen/html/email.html.twig
https://github.com/escopecz/mautic/blob/ed6a9bc33cd01217c6609f4d406493cc8f0a97dd/themes/oxygen/html/page.html.twig
4: edit the config.json file and change the name into Oxygen2 (or so)
5: zip files
6: upload and install the theme.

Creating an e-mail with the editor now works again! Thanks!

Just did a fresh install of Mautic. Created an email and change the color of a button in the Builder. Whe ni try to save the changes with 'Close builder ’ the spinner keeps spinning. In the developer console i see ‘libraries.js?v6e18ae33:571 Uncaught InvalidCharacterError: Failed to execute ‘setAttribute’ on ‘Element’: ‘!important;’ is not a valid attribute name.’