Problem

The rendering of HTML emails is rather ugly, with white/brown backgrounds, totally breaking with the theme. It also is surprisingly slow, especially for long threads and emails containing a lot of quoting in HTML.

Solution

There’s a variable mm-text-html-renderer, which by default is set to shr. I prefer using the value w3m, which uses w3m and is much faster and prettier. It displays everything in the current theme, highlights links, and displays styles, e.g. strong/bold.

It also can be set up to honour a fill-column while rendering.

Setup

As already pointed out: w3m needs to be installed. Via your favourite packet manager (e.g. pacman, apt, yum, brew).

(setq mm-text-html-renderer 'w3m)
(setq w3m-fill-column 72)

For more information try SPC h d v and type mm-text-html-renderer and RET. There are other renderers available, which might work even better for you.