This plugin looks like the one I wanted, to be able to set my custom CSS.
But, when I install it throw Jenkins, the file simple-theme-plugin.hpi has a size of 1 byte and could not be installed.
I go to the project page and download version 0.1 (only 0.2 is defined in the plugin page) and installed it manually. Everything works fine like this, except I am notified for a new version, which I do not want to download.

There may be an error during plugin download, is the version correct ?

Etienne Jouvin