Check-in [8d880f0bb4] altered the stylesheets to support checkboxes (actually labels) in the submenu. For this change to appear as intended, the CSS of every Fossil repository must be updated accordingly.
First of all, install and use the latest version of Fossil, otherwise this change doesn't affect you anyway.
If this is your project's official, central repository server, use the skin setup page to reinstall the skin. Alternately, use the baseline diff feature of the CSS editor to see and incorporate the CSS updates into your custom skin.
If this is a cloned repository, use the fossil config pull skin command to get the updates from the central repository.
The process may be simple to execute once, but the challenge is making the change propagate everywhere. Each user needs to do it. The central server cannot push configuration changes to clones, and the clones do not pull configuration changes through autosync.
Perhaps there needs to be some discussion about how to handle this properly.
The CSS change itself is possibly preliminary since the checkboxes themselves are not styled, and I did just the bare minimum to get the labels to appear like the submenu elements they replaced. More work can be done to get a nicer result, though styling checkboxes is perilous due to browser incompatibility. The point is any effort expended today on propagating CSS updates may have to be repeated soon.