My company's logo, like Atlassian's, is not round. The workaround posted here does not seem to work:
https://jira.atlassian.com/browse/CONF-28275
I put this in the global stylesheet:
.acs-side-bar-space-info .avatar-img-container .avatar-img-wrapper,#image-holder .small-logo-container .small-logo-wrapper {border-radius:0;}
.global.logo, .space.logo {border-radius:0;}
.theme-documentation .space-logo .logo {border-radius:0;}
.project-dialog-header-wrapper .project-header img {border-radius:0;}
and this in the documentation theme header:
.theme-documentation .space-logo .logo {border-radius:0;}
but the logo is still rounded.