Global Styles: Change the behaviour of the reset button

[scruffian]

Is your feature request related to a problem? Please describe.
When I "reset" a style defined in the block defaults, it totally removes the style.

Describe the solution you'd like
I would prefer that the reset button restored the defaults for the block

Describe alternatives you've considered
The current behaviour isn't ideal, as once the setting is changed its impossible to get back to the state intended, unless you happen to remember the values.

[mtias]

Somewhat related, we discussed with @nosolosw and @jorgefilipecosta about the multiple levels of "user customizations" and "theme tweaks" and how they could be presented. Related #27474.