You really ought to move the JavaScript to just before the </body> tag where it belongs, get rid of the 200 year old language attribute off of the script tag and put the JS in a separate file instead of hard coding it in the page.
Style sheet switchers rely on the 3 (as in your example) or 300 (as is possible) different sets of styles being in separate files for the switcher to work so any attempt to make a mess of the code by mixing the CSS with the HTML and JS isn’t going to work.