Fix Dark Mode for custom project picture selection dialog
Summary:
Do not hardcode the icon foreground color `#e7e7e7` but use a similar color already existing as variable `{$lightgreybackground}`.
Refs T15056
Test Plan: * Go to http://phorge.localhost/project/picture/1/ and under "Custom", click "Choose Icon and Color..."
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/D26376