Get default/system theme values

[cwichel]

Is your feature request related to a problem? Please describe. Unable to find a reference on how to read a value from the current default/system theme. For example, I want to read the value of mvStyleVar_FramePadding

Describe the solution you'd like If there's a method already in place, an example of how to do it. If not, using get_item_theme(0) could return the current default/system theme dictionary from where to extract the value

Describe alternatives you've considered None

Additional context None