Where do I set the default stylesheet for the admin section of Joomla, so that this blue block is white?

Over the years I've come to realise my life is a Dilbert sketch
Where do I set the default stylesheet for the admin section of Joomla, so that this blue block is white?

This solves it, for anyone interested.
Go to Mambots,
Site Mambots,
TinyMCE (if that's your editor)
Custom CSS class: location on server
/* CSS Document */
/* Editor Assigned Styles */
/*make sure that any styles you put here also exist
in your template_css.css file or they will have no
effect in the front end */
p.rightalign {
text-align: right;
}
.red {
color: #c00;
}
.green {
color: #0c0;
}
.highlight {
background-color: Yellow;
color: Blue;
padding: 0;
}
p.code {
background-color: #ddd;
border: 1px solid #bbb;
}
.small {
color: #999999;
font-size: 10px;
}
/* Body Tag for Background Color and Font Specs. */
body {
font-family: Arial, Helvetica, Sans Serif;
font-size: 12px;
color: #333333;
background: #ffffff;
}
/* Optional Basic Stuff you can use to make everything look like it does on the site itself */
/* you can get this stuff right out of your template_css.css file, or you can just leave it out */
ul
{
margin: 0;
padding: 0;
list-style: none;
}
li
{
line-height: 16px;
padding-left: 16px;
padding-top: 0px;
/*background-image: url(../images/arrow.png); /* use the real image path that you have on your site. An absolute path is OK */
/* background-repeat: no-repeat;*/
background-position: 0px 2px;
}
td {
text-align: left;
font-size: 12px;
}
a, a:link, a:visited {
color: #000000;
font-weight: bold;
}
a:hover {
color: #000000; text-decoration: none;
font-weight: bold;
text-decoration: none;
}