You can't, you're using OnDemand, which means you can't hack the underlying CSS.
If you've got your own installation, you'd be able to fiddle with the css in there. If your improvements really make it less ugly, it would be nice to share it with Atlassian to see if they'd incorporate any of your prettying.
I’m a designer on the Jira team. For a long time, I’ve fielded questions from other designers about how they should be using Jira Software with their design team. I’ve also heard feedback from other ...
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