Rated 4 out of 5 stars

Have tried your suggestions. Version 30.31 (without Lightning Design) allows me to change event colours using Calendar Tweaks. Don't know how to access 31.8 or 31.8.5 so cannot try these. I am able to change colours in events using all other Thunderbird themes except Australis 31.7.5. I think there is a bug within 31.7.5 that does not allow changes in colours of events - they always stay green! Thank you for your help.

This review is for a previous version of the add-on (31.7.5).  This user has other reviews of this add-on.

Cloud:

https://onedrive.live.com/?cid=ac1c0df493833b9c&id=AC1C0DF493833B9C!125

or look here:

https://onedrive.live.com/?cid=AC1C0DF493833B9C&id=AC1C0DF493833B9C!119

click on the add-on theme version and now download started.

Add-on Theme with versions-number x.x.5 with Lightning-Design, Themes with number x.x nur Thunderbird - Design.