Microsoft Portal Themes

Hi,

I am relatively new to Microsoft portal.

I wanted to change the colour scheme of the default portal, I have read https://docs.microsoft.com/en-us/dynamics365/customer-engagement/portals/create-theme

but this doesn't seem to work. I download the bootstrap.min.css file from Bootswatch.com.

Ive tried amending the current bootstrap.min.css file as well as uploading a new one, and both don't pull through the new styling, am I missing a step?

Thanks

Damesh