Hi

the file is umbraco/assets/css/umbraco.css but don't change this file - it is compliled and minified, and any upgrade of umbraco will break your changes.

instead.

create a folder called backofficestyles in the App_Plugins folder

in that folder place the following package.manifest file.

{
    "css" : [ "~/App_Plugins/BackofficeStyles/backoffice.css" ]
}

Then create a stylesheet called backoffice.css in the folder and make all your changes in that.

the example below will make the sidebar blue:

ul.sections {
    background-color: #0D47A1;
}

ul.sections li.current {
    background-color: #039BE5
}

this will persist between any updates, and you won't need to alter any of the core files.

- When changing style-sheets and settings (which ever way you do it) it is best to have debug="true" on the compilation section of your web.config, and have something like chrome developer tools open on your browser so it doesn't cache the files