File Save dialog unreadable
Can you supply a screenshot? Greyed-out files in file dialogs usually mean that those items can't be selected. For an Open dialog, greyed files are ones than can't be opened by the app.

For a Save dialog, it's normal for existing files to be greyed out, because you're not going to select them, you're creating a new file. (However, you can actually select them to inherit their filename.)

There should be no difference in Light Mode between Mojave and previous versions of MacOS.

There are settings in System Preferences > Accessibility > Display that increase the contrast and Reduce Transparency, which you may have previously used.