Typo3 7: modify the backend login box

Do the following to set a custom logo in the backend login box and to change that logo via CSS:

Extensions -> Typo3 Backend -> configure: define a logo

Then add the following to the ext_tables.php of an extension:

$TBE_STYLES['inDocStyles_TBEstyle']='
.typo3-login-logo {
    background: #000000;
}
';