Re-initialization cause multiple 'Back' links
New variable $topbar-button-top

It's impossible (or I didn't get it right) to center horizontally a button inside a topbar with a custom defined height.

Added a new variable $topbar-button-top to fix this issue
Allow change of topbar link background color

Added a new variable $topbar-link-bg
(usefull if we want transparent link :))
Merge pull request #4136 from sjonnet19/patch-1

Re-initialization cause multiple 'Back' links
Merge branch '5.0-add-topbar-link-color'
Merge branch '5.0-add-topbar-button-top'