Sure, you can create you own custom theme for the Backend. You should then register it in Sitefinity on ~/Sitefinity/Administration/Settings/Advanced >> Appearance >> Backend themes
and then you create new theme by clicking on the Create New
it let's say CustomTheme and in Path
give the path where you put your modified CSS. One you've saved the changes go back to ~/Sitefinity/Administration/Settings/Advanced >> Appearance
and in the BackendTheme
textbox fill in the name of your custom theme (e.g. CustomTheme).
You can take a look at our Eduction Starter Kit
where we have a custom theme applied. You can find the CSS for the customized BackenTheme which you can modify to create your custom one.
the Telerik team