When you modify a parameter in the "Design" tab of a template in Content Builder (i.e. button background color), where is this stored? It doesn't seem to be in the actual code version of the template itself, which is very strange.
If you are using the standard SFMC Templates, then the styling is written in-line with the components e.g. button (as per screenshot)
Similarly, if you are using a Themed SFMC Template, the styling is also written in-line with the components.
The CSS doesn't appear to be in the Template design, but does appear to be controlled somehow by a Theme style guide, as every time I drag in a new component e.g. Button, then it takes the style of the template/other buttons. Subsequently to then change the style of the button, I have to do it individually.
Editing the core Themes/Templates doesn't appear to be an option i.e. to change the style-guide, but you can edit the individual components.
If you are wanting to create a customised style/theme then you could create your own version of a template and create a CSS stylesheet to control any Button classes.