New default theme popups blend into background

[mikest]

Tested versions

branch: 250ef8d

System information

Windows 11, godot 4.6 local build

Issue description

Accessibility and Usability problems

1. Pop-up menus here lack contrast with the background making them difficult to read.

2. Labels in graph nodes should not have borders. White boxes around text convey the wrong UX behavior and make readability poor.
3. Graph nodes should contrast with canvas background. They do not.
   (Both issues fixed by Update VisualShader on theme changes #112382)

Steps to reproduce

Open any popup menu.
Create a shader with a shader graph.

Minimal reproduction project (MRP)

n/a

[YeldhamDev]

The popup problem would be alleviated by #106324.