Using the Task Scheduler, you can schedule when to enable Dark Mode for apps by using these Powershell commands and setting them to be triggered at the time you prefer.
New-ItemProperty -Path HKCU:\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize -Name AppsUseLightTheme -Value 0 -Type Dword -Force
New-ItemProperty -Path HKCU:\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize -Name AppsUseLightTheme -Value 1 -Type Dword -Force;
Source: Windows Central, 2019