Use universal variable for body margin

[jacobkeeler]

Fixes #489

Testing Plan

• Edit $body-margin and verify all overlays (Alert, ScrollableMessage, etc.) keep the same margin as the main body

Core Tests

Tested with $body-margin: 0px and verified that margin was removed from all overlays

Core version / branch / commit hash / module tested against: develop Proxy+Test App name / version / branch / commit hash / module tested against: SDL Test Suite

Summary

Store body margin in universal variable for easy modification

Changelog

Bug Fixes

• Ensure that body margin stay the same for all overlays

CLA

• [x] I have signed the CLA