Get default/system theme values

**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