Fix SharedDecksActivity toolbar color for dark themes

[lukstbit]

Purpose / Description

Saw this when I fixed the empty cards report appearance in dark themes.

Light and plain, no change:

Before/after:

Black:

Dark:

How Has This Been Tested?

Verified the toolbar's appearance in all themes.

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner