Go to Slimstat > Settings > Reports tab and paste your custom CSS into the corresponding field. Use the following code as a reference:

[id^=slim_] .header.is-search-engine, .is-search-engine{
    background-color:#c1e751;
    color:#444;
}
[id^=slim_] .header.is-direct, .is-direct{
    background-color:#d0e0eb;
    color:#111;
}
[id^=slim_] .header.is-known-user,.is-known-user{
    background-color:#F1CF90;
}
[id^=slim_] .header.is-known-visitor,.is-known-visitor{
    background-color:#EFFD8C;
}
[id^=slim_] .header.is-spam,.is-spam{
    background-color:#AAB3AB;
    color:#222;
}