This is using Modern Themes, a variable for theme, and changing that variable with a drop down. Microsoft have said there’ll be Dark Themes in future… but no trace yet.
App.OnStart
Set(varTheme,'Power Apps')
App.Theme
varTheme
Insert DropDown
Items
["Power Apps","Teams","Sharepoint","Steel","Red","Orange","Green","Platinum"]
OnChange
If(Self.SelectedText.Value="Power Apps",Set(varTheme,PowerAppsTheme));
If(Self.SelectedText.Value="Teams",Set(varTheme,TeamsTheme));
If(Self.SelectedText.Value="Sharepoint",Set(varTheme,SharePointTheme));
If(Self.SelectedText.Value="Steel",Set(varTheme,SteelTheme));
If(Self.SelectedText.Value="Red",Set(varTheme,RedTheme));
If(Self.SelectedText.Value="Orange",Set(varTheme,OrangeTheme));
If(Self.SelectedText.Value="Green",Set(varTheme,GreenTheme));
If(Self.SelectedText.Value="Platinum",Set(varTheme,PlatinumTheme));