Firstly there are these instructions:
http://time.ly/document/user-guide/customize-calendar/create-new-calendar-theme/

They dont make much sense or seem to work.

 

OK REVISED

The above does not work – as the calendar updates and overrises what i have done

seems the instructions are right after all ( one of them anyway).

Folders go in wp-content/themes-ai1ec/mytheme

 

What i found worked was this:

I copied Vortex theme and renamed it ‘mycaltheme’ and went went here:
wp-content/themes-ai1ec/mycaltheme/

My Stylesheet (has to be override.css) went here:
wp-content/themes-ai1ec/mycaltheme/css/override.css

My edited Twig files went here:
wp-content/themes-ai1ec/mycaltheme/twig

i did add a blank function file – not sure if it helped:
wp-content/themes-ai1ec/mycaltheme/functions.php

After that things started to work – except my css file.

Every time i uploaded override.css i had to resave Events>Theme Options>Save Options

which was boring

(why cant designers stick with standard themes??)