In order to achieve customization of key tips, you can subscribe to the KeyTipShowing event. In the handler, you can change the BackColor, the BorderColor, the ForeColor or the Font of the key tip. Here is the code sample how can be implemented: 

private void RadRibbonBar1_KeyTipShowing(object sender, CancelEventArgs e)
{
    RadKeyTipShowingEventArgs args = e as RadKeyTipShowingEventArgs;

    args.BackColor = Color.FromArgb(125, Color.LightGreen);
    args.BorderColor = Color.Black;
    args.ForeColor = Color.Red;
    args.Font = new Font("Calibri", 25f, FontStyle.Bold);

    if (sender == this.radButtonElement1)
    {
        args.CustomLocation = new Point(20, 80);
    }
}

This customization will be available in our next release - R1 2018, scheduling for January 2018.