Welcome Screen updates + theme fixes

[crocsandcoffee]

📑 What does this PR do?

• UX updated to the Welcome Screen to get us started
• Updated Desktop app root to Welcome Screen
• Theme fixes
• Added placeholder app icon. (Pulled from Icons8)

✅ Checklist

• [ ] My pull request adheres to the code style of this project
• [ ] My code requires changes to the documentation
• [ ] I have updated the documentation as required
• [ ] All the tests have passed

🧪 How can this PR been tested?

Run the android and desktop app

🧾 Tasks Remaining: (List of tasks remaining to be implemented)

• Hook up the SignUp and LoginIn button to new screens

• Figure out how we can share an app icon image between platform modules

• What is remaining to be implemented in this PR? Mention a list of them

🖼️ Screenshots (if applicable):

[crocsandcoffee]

If the ic_elephant and the placeholder image url do not pose a risk regarding their license, I'm fine

It's pulled from icons8. https://icons8.com/license

But just to be safe I'll replace it with a built in icon from material before I merge. Same with the image url.