Updated default styling of horizontal rule <hr>

What does this MR do?

• Updates the default styling of horizontal rule

  ---

  from 24px 0 to 1rem 0

Fixes #293926

Screenshots (strongly suggested)

NOTE: I'm submitting this as a part of GitLab Hackathon!

Does this MR meet the acceptance criteria?

Conformity

• Style guides

Thank you 😄 /cc @cbacharakis

UPDATE : 16.01.21

• The size of <hr> tag has been increased to 1.5rem from 1.0rem following this.

• Screenshot :~