I confirm the same issue. When I am in the /portal/admin/settings/menu-settings I cannot switch back to Light Mode sometimes but once I refresh with F5 then I can switch it back to the Light Mode.

To replicate this issue please check case scenario:

As the Admin user go to #Manage > Settings > Menu Settings > try to Add/Edit a link in the Sidebar Bottom Link Groups. Once the right popup pane is being opened and closed then you wont be able to Switch to light mode etc.

It looks like when the popup window is being opened even just for 1 sec and closed then there is the issue with switching to Light Mode (Z-index issue?)