PM-10281: Set custom app theme on splash window

[matt-livefront]

🎟️ Tracking

PM-10281

📔 Objective

Applies an overridden light/dark mode theme to the splash screen. There's still a brief portion of the app launch that we can't override, but the splash screen should have the correct theme after that.

📸 Screenshots

App launch:	Before	After

App switcher:	Before	After

⏰ Reminders before review

• Contributor guidelines followed
• All formatters and local linters executed and passed
• Written new unit and / or integration tests where applicable
• Protected functional changes with optionality (feature flags)
• Used internationalization (i18n) for all UI strings
• CI builds passed
• Communicated to DevOps any deployment requirements
• Updated any necessary documentation (Confluence, contributing docs) or informed the documentation team

🦮 Reviewer guidelines

• 👍 (:+1:) or similar for great changes
• 📝 (:memo:) or ℹ️ (:information_source:) for notes or general info
• ❓ (:question:) for questions
• 🤔 (:thinking:) or 💭 (:thought_balloon:) for more open inquiry that's not quite a confirmed issue and could potentially benefit from discussion
• 🎨 (:art:) for suggestions / improvements
• ❌ (:x:) or ⚠️ (:warning:) for more significant problems or concerns needing attention
• 🌱 (:seedling:) or ♻️ (:recycle:) for future improvements or indications of technical debt
• ⛏ (:pick:) for minor or nitpick changes

[github-actions[bot]]

Checkmarx One – Scan Summary & Details – e5166f81-f29e-419a-a40e-ee2bf921916c

No New Or Fixed Issues Found

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.10%. Comparing base (cc8039a) to head (34ee53f). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1008 +/- ## ======================================= Coverage 89.10% 89.10% ======================================= Files 657 657 Lines 41317 41319 +2 ======================================= + Hits 36814 36816 +2 Misses 4503 4503 ```

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