Update design to use new styleguide (bootstrap 4)
https://styleguide.torproject.org/ provides a Bootstrap fork customised for Tor Project to provide a consistent visual identity across subprojects. Bootstrap 3 is no longer maintained and at some point we will need to update to Bootstrap 4 anyway to avoid any brokenness in browsers that may appear (we have already seen brokenness that we've had to patch manually in #25254 (moved)).
While implementing this, we may also choose to make other design changes or to reorganise navigation on the site. It would probably be worthwhile to hold off making any large changes to the current structure of the site until it has been considered how it would work with the new styles.