After upgrading Simple Theme Plugin from version 0.4 to 0.5 on Jenkins ver. 2.107.3, the following simple CSS rule saved in the Theme global properties no longer works. I tried deleting and recreating the rule with no success.
The CSS rule suppresses pipeline output, as recommended in a comment on this issue: JENKINS-41845
Developer Tobias Gruetzmacher further commented at https://wiki.jenkins.io/display/JENKINS/Simple+Theme+Plugin :
This is indeed a bug, introduced by the refactoring work between 0.4 and 0.5: While https://github.com/jenkinsci/simple-theme-plugin/blob/simple-theme-plugin-0.4/src/main/resources/org/codefirst/SimpleThemeDecorator/header.jelly#L9-L11 and https://github.com/jenkinsci/simple-theme-plugin/blob/simple-theme-plugin-0.5/src/main/java/org/jenkinsci/plugins/simpletheme/CssTextThemeElement.java#L12 look the same, it seems Jelly helpfully removed the CDATA tags before, which it can't anymore, since the tags are inserted raw.