Control Panel CSS is throwing error “Use of undefined constant PATH_CP_THEME”

I’m attempting to include custom CSS in the control panel, but the CSS file “admin.php?/cp/css/cp_global_ext” simply shows an error and doesn’t output any CSS:

Use of undefined constant PATH_CP_THEME - assumed 'PATH_CP_THEME'
ee/legacy/conotrllers/cp/css.php, line 73