Styling jenkins

You can customize the html or css files in war sub folder.

Or, can update them using below pugin.

https://github.com/jenkinsci/simple-theme-plugin
https://plugins.jenkins.io/simple-theme-plugin/
https://medium.com/@elhayefrat/replace-jenkins-logo-and-text-to-your-choice-in-jenkinsui-f7d35daed25b

samples:
http://afonsof.com/jenkins-material-theme/