The theme.css is found in the theme module i think. i've not really used it.
You can also inject it in a style class which will still work on every dropdown.
If you arent used to css or dont not really gonna use it that often this should work for now.
}.iaDropdownCommon_options_modal { max-height: 300px; }{
(300px or whatever you wish, 150px is standard atm)
more info i posted here:
edit newer versions +8.1.22(?):
add >div:first-child
and !important
Also suggested to use the new feature stylesheet.css instead of the old hacky css injection
https://docs.inductiveautomation.com/display/DOC81/Style+Classes#StyleClasses-EnablingtheAdvancedStylesheet