It's on purpose to discourage that type of submissions. The links in the text are not converted to real links. And also, these submissions have a penalty so it's more difficult for them to reach the front page. If possible, I recommend using a normal submission.

Anyway, if you want an official reply from the mods, try writing an email to hn@ycombinator.com

WAVE (Web Accessibility Evaluation Tool) shows numerous contrast errors[1]. (Click the contrast button on the sidebar.)

[1] http://wave.webaim.org/report#/https://news.ycombinator.com/...

You gain flexibility by letting your browser handle such things. Have a look at Stylus https://github.com/openstyles/stylus, which is a privacy-conscious fork of Stylish for Chrome, also compatible with Firefox as a WebExtension.

probably not, don't think they make frequent updates.. but you can use stylish plugin to change CSS on hackernews or any site really.