Open System Settings
Click Appearance
Select Dark
Turn on Auto if you want it to switch at sunset
Open Displays
Reduce Brightness
Turn off True Tone if the screen looks too warm
Turn off Night Shift if you want a cooler dark look
Increase Contrast in Accessibility
Open Accessibility
Click Display
Turn on Increase contrast
Turn on Reduce transparency
Use a darker wallpaper
Set the Dock to Dark
Open Desktop & Dock
Turn on Automatically hide and show the Dock
Use a dark menu bar by keeping Dark appearance enabled
Lower keyboard backlight if needed
Use a darker app theme in apps that support it
Adjust external monitor brightness if using one