Download “is_styleswitcher” from Typo3 Extension Repository

Install the extension

Set Typoscript-Setup (see Example below)  (TODO: automatically detect filepath)

plugin.tx_isstyleswitcher_pi1.absCSSFilePath = /fileadmin/template/main/_css

Include the Extension through a Typoscript-Marker (check Documentation  “Modern Template Building” for basic information on Typoscript and “Futuristic Template Building” for further Information on TemplaVoila). For “Normal Template” and “Autoparser-Extension”:

temp.myMarker < plugin.tx_isstyleswitcher_pi1

For “TemplaVoila Templates”:

lib.myMarker < plugin.tx_isstyleswitcher_pi1

Don't mix with manually integrated CSS-files (via Template or Typoscript) as they will be not recognized by the extension and will be always integrated. If they are included after the extension-code, they might override the CSS-styles set by the this extension.

Try to avoid using the inline-CSS created by Extensions. Put these into external files  and override the extension-css by setting plugin.tx_<extensionkey>._CSS_DEFAULT_STYLE = Include these external files in the Switcher-Extension.