The editor keyword colors in light mode could be incorrect after using Beta 3 or earlier. The "Symbol" color was light blue (cyan), and the "Keyword" color was brownish. To fix this you can go to Preferences > User Interface > Editor and click on the green indicator of the "Syntax highlighting" color section. This will revert the editor to the correct default colors.

In dark mode the default editor colors are correct.

You may need to do the same for the Browser, Grid and Editor fonts on the Preferences > Fonts page.

To fix all colors, you can do the following:

1. Download this light.zip file.
2. Exit PL/SQL Developer.
3. Go to %APPDATA%\PLSQL Developer 15\Preferences\Themes
   (e.g. C:\Users\<your user name>\AppData\Roaming\PLSQL Developer 15\Preferences\Themes)
4. Unzip the downloaded light.ini file to replace the existing file.
5. Restart PL/SQL Developer.