How do I configure style sheet?

In version 3.03, I had to modify atlassian-jira\template\includes\issuetable\issue.jsp
(line 90 changed width from 1 to 100 for description)
and issue_basic.jsp
(line 27 changed width from 100 to 40 for summary)
With the improvement JRA-6227, how do I now replicate this?
I could not find ay documentation on this new feature.

Thanks