They could provide style sheets, but you would need to place them at a level which would affect ALL Perspective projects on the Gateway.
They could easily use a secondary Designer to design Styles classes while you develop the HMI screens, but you’re going to run into a barrier of sorts; Perspective is designed from the ground up to hold your hand and make Views easy to design by doing a lot of the “Page” layout and design heavy-lifting for you, and we do NOT expose any way to manipulate Components/Containers/Popups/DockedViews outside of what you see in the Designer.
So consider a basic Button… We provide hundreds of options for styling and resizing and positioning and function, but a web-developer is going to see that they have no say in how the actual HTML for the button is constructed, and they will probably be turned-off, because they’re limited in what they can manipulate.