Here’s how to dynamically change a web page’s colors using the CSS filter property and JavaScript:
<a href='javascript:document.querySelector("html").setAttribute("style","filter: saturate(60%) brightness(80%);")'>
pastel
</a>
<a href='javascript:document.querySelector("html").setAttribute("style","filter: grayscale(100%); ")'>
grayscale
</a>
<a href='javascript:document.querySelector("html").setAttribute("style","filter: ")'>
normal
</a>