Sec. 38-241. - Prohibition on the open carrying of firearms.
The City of Thornton hereby prohibits the open carrying of a firearm, as defined in Section 38-237 of the Code, in any city building or in or upon any city property including but not limited to parks, open spaces or trails that exist within the City of Thornton and that have a sign posted at the entrance to any city building or city property informing persons that the open carrying of firearms is prohibited in such building or area.
Looks legal to me.