This Customadmin plugin for Grav is, in fact, only few lines that add a CSS and a JS file to the Grav Admin panel. The files are included with the lower priority in order to let you simply override everything you need to customize.
First ensure you are running the latest Grav 1.0.10 or later and the latest Grav Admin Plugin 1.0.9 or later. This is required for the customadmin plugin to run properly.
Put the content of this repository in a folder called "customadmin" in :
user/plugins/
Update this two files to customize the admin panel as you want :
user/plugins/customadmin/customadmin.css
user/plugins/customadmin/customadmin.js