Workboard: in some cases, increase menu opacity (accessibility)

Summary:
This change contains a very minimal graphical change for Workboards
with a custom background.

https://we.phorge.it/T15186

After this change, if your Workboard has a custom background color,
the opacity of the floating menu of the Column is not 0.9 but 0.95,
so it's increased a bit. The problem with the previous value, is that
when you open the menu, you see too much text underneath, and this
could worsen the readability. Now also probably, but less I hope.

Closes T15186

Test Plan:
- Workboard > Change Background Color > Pick a nice color
- Backlog > Edit
- Enjoy the extra 0.05 of opacity of that menu
- NEVER ACCEPT IF YOUR EYES DETECT JUST A 0.04999998 OF EXTRA OPACITY

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15186

Differential Revision: https://we.phorge.it/D25088