Nothing is changed on the software itself: It's your broswer that applies more CSS rules to make it happens. It relies on the Chrome extension xStyle, and that make it easy to turn them on or off, just in case it conflict with the website.

Install xStyle from the Chrome Web Store ⎋
In the extension, create a new Style
Copy one of the files' content available here, in the text editor
You can use the file name as the style name, or use your own.

Repeate for each styles you want to enables.

What each styles do?

PREP banner.css: Change the Preproduction banner and login's footer
PLLR banner.css: Change your Pillar banner and login's footer
PERF banner.css: Change the Load Testing banner and login's footer
PROD banner.css: Change the Production banner and login's footer
Whiter banner.css: Look & feel, mainly changing text to pure white and the hover and active elements. A must have. Works with the others files.
Sigils.css: Change the top-right user icon to a Shop or Factory depending on if you are logged as ar Retailer or a Supplier.
Jirafy.less: Animate the Jira navbar with a light rotation on buttons. Lighten the Create button as well.
Navbar color on Notif.css: Emphase the notification banner by coloring the navbar as well, with a nice and tidy transition.