SOLIDPOD: fix loading indicator background in GrantPermissionUi()

[jesscmoore]

Describe the Bug

When the GrantPermission() is loading data, it shows a loading indicator which is not theme responsive. In night theme, the background behind the loading indicator is white.

The background behind the BACK button should also be fixed to match the theme responsive background of the rest of the page.

To Reproduce

On notepad,

1. Click into night mode
2. Click share icon on a note to open sharing page. While it is loading we see the different background behind loading indicator.

Expected Behaviour

Background behind loading indicator and back button should match the page background and be responsive to theme change like the page background

Screenshots

Context

Where has the issue been observed:

• Android
• Chrome
• iOS
• Linux
• macOS
• Web
• Windows

App Version: solidpod 0.9.8
Flutter Version: 3.38.7

Closing Criteria

Checklist for closing the issue:

• No errors from make prep
• All tests pass make qtest
• Background behind loading indicator and back button match the rest of the GrantPermissionUI background.