title: Change ComboBox border outline color
body: <p>I am trying to manage the color of the ComboBox. While it is possible to change the background color, I cannot find a property for the border outline.</p>
  
  <p>Just drawing a square as border wont do in a dark theme because of the arrow. This leads me to the conclusion that this border might be an actual image file.</p>
  
  <p>Is it possible to replace this?</p>
  
  <p><a href="https://i.stack.imgur.com/AV4rH.png" rel="noreferrer"><img src="https://i.stack.imgur.com/AV4rH.png" alt="enter image description here"></a> </p>
  
  <p><strong>UPDATE:</strong>
  I have implemented the Solution of @AhmedAbdelhameed - It looks a lot better now. However for the flat style, I had to adjust the rectangle like the following:</p>
  
  <pre><code>using (var p = new Pen(this.BorderColor, 1))
  {
      g.DrawRectangle(p, 0, 0, Width - buttonWidth - 1, Height - 1);
  }
  </code></pre>
  
  <p>I also exchanged the 'BorderColor' to match the rest of my UI:</p>
  
  <pre><code>public CustomComboBox()
  {
      BorderColor = Color.Gray;
  } 
  </code></pre>
  
  <p>This is the result so far:
  <a href="https://i.stack.imgur.com/6T2nj.png" rel="noreferrer"><img src="https://i.stack.imgur.com/6T2nj.png" alt="enter image description here"></a>
  <a href="https://i.stack.imgur.com/LBuru.png" rel="noreferrer"><img src="https://i.stack.imgur.com/LBuru.png" alt="enter image description here"></a></p>
  
  <p>What I would like to be able to do now is to change the actual drop down button (maybe with an overlay png) <em>only in dark theme</em></p>
  
  <p><strong>UPDATE:</strong>
  I have been able to add a pricturebox to the custom Control with the following code:</p>
  
  <pre><code>using (var g = Graphics.FromHwnd(Handle))
  {
      using (var p = new Pen(this.BorderColor, 1))
      {
          g.DrawRectangle(p, 0, 0, Width - buttonWidth - 1, Height - 1);
      }
      if (Properties.Settings.Default.Theme == "Dark")
      {
          g.DrawImageUnscaled(Properties.Resources.dropdown, new Point(Width - buttonWidth - 1));
      }
  }
  </code></pre>
  
  <p>it looks awesome!
  more or less by coincidence which I do not understand, the dark dropdown button even disappears, when I change the theme in the theme combobox.</p>
  
  <p><strong>Before - After Comparisation:</strong>
  <a href="https://i.stack.imgur.com/AV4rH.png" rel="noreferrer"><img src="https://i.stack.imgur.com/AV4rH.png" alt="enter image description here"></a> 
  <a href="https://i.stack.imgur.com/x2gH4.png" rel="noreferrer"><img src="https://i.stack.imgur.com/x2gH4.png" alt="enter image description here"></a></p>