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?

PPR banner.css: Change the Preproduction banner and login's footer
PLR banner.css: Change your Pillar banner and login's footer
LTG banner.css: Change the Load Testing banner and login's footer
PRD 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.
Alkessian.less: Change the black navbar of Jira & Confluence back to the default white one and add slight animations.