This resolves bug 414 https://bugs.hamaralinux.org/show_bug.cgi?id=414

- added the welcomeStyle option in branding, explicitly

setting the security repo from buster to testing. So we don't have to keep changing them every time for newer releases

- The Calamares Installer now follows the Hamara style guide for the
colours on the sidebar

only till we get new backgrounds.