How to change System.Windows.Forms.ToolStripButton highlight/background color when checked?

You can provide your own tool strip renderer to draw the button's background the way you want them. This example code gives the checked button a very visible black background:

public partial class Form1 : Form {
    public Form1() {
        InitializeComponent();
        toolStrip1.Renderer = new MyRenderer();
    }
    private class MyRenderer : ToolStripProfessionalRenderer {
        protected override void OnRenderButtonBackground(ToolStripItemRenderEventArgs e) {
            var btn = e.Item as ToolStripButton;
            if (btn != null && btn.CheckOnClick && btn.Checked) {
                Rectangle bounds = new Rectangle(Point.Empty, e.Item.Size);
                e.Graphics.FillRectangle(Brushes.Black, bounds);
            }
            else base.OnRenderButtonBackground(e);
        }
    }
}