Add experimental padding support to Varia

[jordesign]

Changes proposed in this Pull Request:

Adds theme support for 'experimental-custom-spacing' to Varia themes

Related issue(s):

Addresses (part of) #2277

[andres-blanco]

Hi @jordesign ! I applied this change but I don't see any change. Is there anything special that should be done to enable the experimental flag?

[kwight]

@jordesign Could you add some testing instructions please?

[jordesign]

I'll double check and then add some instructions