Visible will default to your system settings, but you can override this by enabling dark mode directly in the solution. This article walks through how to do this:
Step One: Select your user profile at the bottom left side of your screen. Then click My account.
Click your user profile icon at the bottom left side of your screen. Then select My account.
Step Two: Click Preferences from the inner navigation menu. Then, click Dark mode under your theme settings.
Select Preferences from the inner navigation menu. From there, click Dark under the Theme settings.
Let us know if you have any questions!