Disneyland *had* a theme in the same way MK has a theme and DLP, TDL, etc have a theme. Adventure, Fantasy, Tomorrow, America's Frontier, and Southern US/New Orleans.
DL sort of lost it's original theme with Galaxy's Edge and to a lesser extent Mickey's Toontown, although I feel like that fits decently as an extension of Fantasyland. Galaxy's Edge just feels like it doesn't belong at all (although oddly I think it transitions between Critter Country to SWGE to Frontierland better than SWGE transitions between lands at DHS.