Tweak new theme

[seadowg]

Closes #4832

As well as fixing the problems in the issue this makes the back/next button bar more visible. This should really use a BottomAppBar which would give us the correct color based on elevation but that's a switch we don't need to dive into right now.

What has been done to verify that this works as intended?

Quick play with the app

Why is this the best possible solution? Were any other approaches considered?

The only real discussion here is around the nav bar, and I've covered that above.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Should just fix the issues! I think this can go in without QA as it's just some small fixes.

Before submitting this PR, please make sure you have:

• run ./gradlew checkAll and confirmed all checks still pass OR confirm CircleCI build passes and run ./gradlew connectedDebugAndroidTest locally.
• verified that any code or assets from external sources are properly credited in comments and/or in the about file.
• verified that any new UI elements use theme colors. UI Components Style guidelines

[kkrawczyk123]

Tested with success!
Verified on Androids 5.1 and 11.

Tested cases:

• default/light/dark mode
• server url
• username
• odk password
• fallback submission url
• project name
• project icon
• hex in project color
• form metadata/username
• form metadata/phone number
• form metadata/email address
• admin password/show admin password
• all widgets
• ex type widgets
• back and next button
• get blank form/ get selected button
• delete saved form

[mmarciniak90]

Tested with success on Android versions: 8.1, 9.0, 10.0