If you want to change CSS, you can use plugin for your webbrowser (i.e. Stylish, Greasemonkey). It won't break things, can be used with OnDemand. But it will only work on your computer.
You could also unpack plugin's jar file and change css file. Just change jar extension to zip and find css file (usually in css directory). Still, after updating plugin, you will lose your styles.
You could also create your own plugin that only adds custom css to JIRA. It's rather hard for average JIRA admin, but if you can create plugins for JIRA, it's easy. Check https://developer.atlassian.com/display/JIRADEV/Web+Resource+Plugin+Module Use "jira.global" context to include your css on every standard JIRA page.
I advice you to browser plugin.
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