Look ma; no CSS
For those enlightened souls who use Firefox: there is a thing called reader-view. It works on pages with a good semantic HTML structure and helps you read without distractions. You might have accidentally clicked this icon in your address bar and noticed everything disappearing but the basic content. Almost as if we're thirty years in the past when CSS didn't exist yet. Except reader-view does look polished, has styling options, and a screen reader.
Way back
That got me thinking: once in a while you come upon these sites that let you change the visual appearance (yeah like darkmode, but different). What would my site look like without styling, in 1995? (CSS1 came out in 1996)
Array.from(document.querySelectorAll('link,style')).forEach(elm=>elm.remove())
Well it looks all wrong and ugly because I was too lazy to add a width and height to the SVG icons. And some of these icons are useless without CSS. Or rather: they were hidden with CSS and have no current function.
Dead kittens
Scaling down the SVG's is no sweat. But hiding the SVG's that would have been hidden by CSS takes a bit more effort.
Best would be to remove these from the DOM altogether. Easiest is to hide them with inline styling and override that display in the stylesheet. Sad thing is: that override will not work without using !important. And everytime you use !important a little kitten dies.
Theming
The link above removes styling but is not persistent when you reload. For that we need to implement this as a theme: so store the state in localStorage and act upon it on page load.
This requires two elements to be manipulated when changing style: set the href property of the HTMLLinkElement to swap the exteral stylesheet, and add or remove the inline HTMLStyleElement. I hope you have the latter because inlining above-the-fold CSS is a major rendering optimisation.
For now I've added simple theming: the default, no theme and a contrast theme (which is really a minimal theme to make no-theme look good).
Expect a bigger theme in the future.
(theme switch in the footer)