This page demonstrates a nifty HTML trick. If you make a style tag contenteditable and display: block; it, you can change the styles for a page live.
For example, try editing the styles for the paragraphs and h1 below:
Since this is CSS that is interpreted by the browser, you can write any valid css, like media queries or pseudo-element selectors.
The CSS Below sets up the style tags. If you find a light theme easier, try changing the background color and font color. Be careful though, if you change the display value here, you loose the ability to view and edit the styles
This is an idea that came up after viewing this brilliant HTML quine. Code is here.