The Mathematica documentation gives this example of exporting an HTMLFragment using the "TemplateEnvironment" options:

SystemOpen @ Export[
     "cellaut.html",
    ArrayPlot[CellularAutomaton[30, {{1}, 0}, 50], Frame -> False], 
    "HTMLFragment", 
    "TemplateEnvironment" -> <|
            "Theme" -> "Cerulean",
           "Brand" -> "MyCompany",
           "PageTitle" -> "Array plot",
           "PageDescription" -> "This is the array plot description",
           "Footer" -> "Nice footer"
        |>
  ]

Changing the "Theme" parameter seems to do nothing. Does anyone know of any additional information on what the theme options are or how to use this to change the appearance of the exported HTML?