Decreased the opacity on the mask for dialogs

Summary:
The transparent mask around the dialogs was too dark. Code was hard to
read. Now opacity has a value of 37, which makes the code easy to read,
but still clearly indicates that the dialog is present.

Test Plan:
I was able to read the code easily through the mask.

Reviewed By: epriestley
Reviewers: epriestley
CC: epriestley, tuomaspelkonen
Differential Revision: 139