Fix #1038 dark mode dialog text color issue

[crvdgc]

Fixed two places where the button colors are hard-coded. Also changed AlertDialog's style so that the positive and negative buttons have proper text colors, instead of black.

Closes #1038.

[oliexdev]

First of all thx for the PR. On which Android API did you tested it? I tried it with API level 33 and 23 and unfortunately it the text on the birhtday date picker on #1038 is still the same.

[crvdgc]

On which Android API did you tested it?

I'm sorry, due to a change of computer, I lost the previous configuration for this project. I think it's automatically picked up by Android Studio, so it's very likely to be the latest one. (Sorry, I'm not very familiar with Android development as well.)

One thing I used to test the PR is to add a manual measurement without creating a user, which triggers this showNoSelectedUserDialog error. Before this PR, the buttons of that dialog in dark mode are black. This PR should at least fix them. Can you test if this is the case? If so, the birthday picker error may be a result of a missing change.