How to update changes in a custom base Mosaico template?
When i was building custom templates, clearing browser cache mostly worked. My favorite is to use Ctrl+Shift+R.

And in the UI - I would try creating new template from the base. 

An already saved template may not reflect your changes, because saved blocks also store css information and may not use the new one. However any new blocks or deleting a save block and re-creating might work.

Also keep an eye on console for any JS errors before and after your changes. Sometimes even simple css changes break the knockout js, and may not reflect.

Hopefully that helps.