I can't seem to find anything at all on the internet related to my problem which makes me think I've done something wrong along the way which has eluded me for days now.
I have a theme I downloaded called 'Ultimo' and I want to make changes to the header.phtml file without changing the core file, so obviously I would make a sub-theme/child-theme.
Inside the Ultimo 'package' it has a default theme called 'default'
To make a sub-theme I need to make a folder alongside 'default' called 'custom_theme' and then enable it within the admin panel, See below.
I delete the cache after any changes I make. There is nothing in system > design that could override anything.
If I rename the folder 'default' to something else ONLY THEN will magento load in the desired sub-theme.
So obviously magento is skipping the sub-theme I created and going straight to default.
Please can anybody help me as I'm not sure what is making Magento behave like this.
(I made a test copy of my site from before I added the new theme and was able to add the theme, create a sub-theme and enable it no problem)