You have to draw it by yourself, because there is not published API for this. Hopefully this is relatively easy to do it in non-visualstyles way.
You can draw pane border with ControlPaint.DrawBorder3D and use something like the following code for buttons:
int Top = bounds.Top;
int Bottom = bounds.Bottom - 1;
int Sign = 1;
if (tabStrip.EffectiveOrientation == TabOrientation.Bottom)
{
Top = bounds.Bottom - 1;
Bottom = bounds.Top;
Sign = -1;
}
using (Pen OuterLightBorderPen = new Pen(SystemColors.ControlLightLight))
{
e.Graphics.DrawLine(OuterLightBorderPen, bounds.Left, Bottom, bounds.Left, Top + 2 * Sign);
e.Graphics.DrawLine(OuterLightBorderPen, bounds.Left, Top + 2 * Sign, bounds.Left + 2, Top);
e.Graphics.DrawLine(OuterLightBorderPen, bounds.Left + 2, Top, bounds.Right - 3, Top);
}
using (Pen InnerLightBorderPen = new Pen(SystemColors.ControlLight))
{
e.Graphics.DrawLine(InnerLightBorderPen, bounds.Left + 1, Bottom, bounds.Left + 1, Top + 2 * Sign);
e.Graphics.DrawLine(InnerLightBorderPen, bounds.Left + 2, Top + 1 * Sign, bounds.Right - 3, Top + 1 * Sign);
}
using (Pen OuterDarkBorderPen = new Pen(SystemColors.ControlDarkDark))
{
e.Graphics.DrawLine(OuterDarkBorderPen, bounds.Right - 2, Top + 1 * Sign, bounds.Right - 1, Top + 2 * Sign);
e.Graphics.DrawLine(OuterDarkBorderPen, bounds.Right - 1, Top + 2 * Sign, bounds.Right - 1, Bottom);
}
using (Pen InnerDarkBorderPen = new Pen(SystemColors.ControlDark))
e.Graphics.DrawLine(InnerDarkBorderPen, bounds.Right - 2, Top + 2 * Sign, bounds.Right - 2, Bottom);