Didier, thank you for your comment. "High Contrast" is very white. There is a "HighContrastInverse" available in Tweaks which is very black. Selecting either of those themes doesn't change the color of the Shell modal dialogs. (An easy way to see the dialog is to click the system status menu in the right side of the top bar and click the Power button at the bottom of the menu.)
Didier, thank you for your comment. "High Contrast" is very white. There is a "HighContrastIn verse" available in Tweaks which is very black. Selecting either of those themes doesn't change the color of the Shell modal dialogs. (An easy way to see the dialog is to click the system status menu in the right side of the top bar and click the Power button at the bottom of the menu.)