Fix dark mode for Differential Paths Panel divider
Summary:
Replace hardcoded background color `#f5f5f5` for the Differential vertical divider (`.phui-formation-resizer`) between the Paths Panel and the actual revision in Differential with variable `{$lightgreybackground}` which is `#f7f7f7` in default mode. That is the same color that the adjacent sidebar (`.phui-flank-view-fixed`) already uses, and I consider the small difference in grey neglectable.
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Create a Differential revision
* Go to http://phorge.localhost/D1
* In the left upper corner, click that arrow thingie to Show Paths Panel
* Look at the vertical divider
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15056
Differential Revision: https://we.phorge.it/D26164