Can you provide more information on what you're asking ?
If you are asking whether its possible to change the Content Style Sheet - then yes, its possible to change that. An example is the color of the header or the layout of the initial login screen.
If you're asking about a specific application within E1 - then you'll have to do that in the JDE Toolset as the code is interpreted at runtime, and what you "see" is how the form is being generated each time you open the form. There is no HTML to change, as it gets generated each time in effect...