For all those who have their settings on low but don't want opaque windows in-game, can you try the following?
- Right click on the Sims 4 in your Applications folder and select Show Package Contents
- Open the Contents folder and then click on the GraphicsRules.sgr file and open with TextEdit
- Scroll down to the settings for low and find EnableOpaqueWindow
- Change the true to false
- Close the file
If your game screws up or doesn't load just go back in and change the value to true.