Make DevSignerScreen use the app theme provider consistently

[manoahLinks]

Summary

Align DevSignerScreen with the rest of the app by using useAppTheme instead of importing the lower-level theme hook directly.

Suggested files

• src/screens/DevSignerScreen.tsx

Acceptance criteria

• The issue described above is implemented or documented in a focused change.
• Existing behavior outside this task is preserved.
• npm run typecheck passes when code is changed.
• UI changes include screenshots or a short manual verification note.

Contributor notes

Please keep the PR scoped to this issue and mention any backend assumptions in the PR description.

[ABEEGOLD]

@ABEEGOLD has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I would like to use my expertise to handle this issue if you generously grant me to.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ABEEGOLD to this issue.

[drips-wave]

Congratulations, @ABEEGOLD! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @ABEEGOLD: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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