As described in https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/checkbox#Indeterminate_state_checkboxes, this feature has been supported in browsers since time immemorial (aka IE6): https://www.caniuse.com/#feat=indeterminate-checkbox
The state is rendered on different platforms as either a hyphen 
(my preference) or a square 
.
The current patch in GlobalPreferences proposes a square: 
(https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/GlobalPreferences/+/459533/)
A hyphen would look something like this: 
