When you customize a theme for RadSplitButton, you are not able to change the color of the action button element when it is pressed and the drop down is not opened. 

Workaround:
this.radSplitButtonElement2.ActionButton.MouseDown+=ActionButton_MouseDown;
this.radSplitButtonElement2.ActionButton.MouseUp+=ActionButton_MouseUp;

private void ActionButton_MouseUp(object sender, MouseEventArgs e)
{
    this.radSplitButtonElement2.ActionButton.ButtonFillElement.ResetValue(LightVisualElement.BackColorProperty, ValueResetFlags.Local);
    this.radSplitButtonElement2.ActionButton.ButtonFillElement.ResetValue(LightVisualElement.GradientStyleProperty, ValueResetFlags.Local);
}

private void ActionButton_MouseDown(object sender, MouseEventArgs e)
{
    this.radSplitButtonElement2.ActionButton.ButtonFillElement.BackColor = Color.Red;
    this.radSplitButtonElement2.ActionButton.ButtonFillElement.GradientStyle = GradientStyles.Solid;
}

Workaround:
 public class MyRadToggleSwith : RadToggleSwitch
{
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadToggleSwitch).FullName;
         }
    }

    protected override void OnGotFocus(EventArgs e)
    {
        base.OnGotFocus(e);
        this.ToggleSwitchElement.Focus();
     }
}

or simply put focus to the element when the control receives the focus:
private void radToggleSwitch_GotFocus(object sender, EventArgs e)
{
	this.radToggleSwitch1.ToggleSwitchElement.Focus();
}

To reproduce: 
1. Open the Demo application >> Button example.
2. Change the theme to TelerikMetroBlue.
3. Click with the right/middle mouse button and you will notice that the button clicked states is indicated.

Workaround: remove the MouseDown state in Visual Style Builder for the fill element as it is illustrated on the attached screenshot.

To reproduce:
- Create a dpi aware application and add a checkbox to it. 
- Change your DPI setting to 200% and start the application.
- The checkmark is not scaled only the text is. 

Workaround:
this.radCheckBox2.ButtonElement.CheckMarkPrimitive.CheckElement.UseFixedCheckSize = false;
this.radCheckBox2.ButtonElement.CheckMarkPrimitive.CheckElement.MinSize = new Size(100, 100);

Steps to reproduce:

1. Press the left mouse button inside RadToggleSwitch. (MouseDown)

2. Move the mouse 1 px up or down (but still inside the RadToggleSwitch). (MouseMove)

3. If you release the left mouse button now, the switch will not toggle. (MouseUp)

Only if the mouse location at the moment of Press and Release are exactly the same, the toggle action will be triggered.

To recreate the problem:

1. Create a new Telerik Winforms .Net Project
2. Add a radButton
3. Change the font property in the designer to a different size

Workaround: change the font for the TextPrimitive:

See attached to reproduce. 

Workaround: set it to the element:

 radToggleSwitch1.ToggleSwitchElement.Font = new Font("Segoe Script", 18, FontStyle.Regular);

To reproduce:
- radSplitButton1.DefaultItem = radSplitButton1.Items[0];
- press and hold the action button
- the styles are not changed

Use attached to reproduce.

Workaround:
radDropDownButton1.DropDownButtonElement.DropDownMenu.MinimumSize = new Size(300,0);

To reproduce:
- Set the theme to FluentDark and the set RightToLeft = Yes.

Workaround:
radToggleSwitch1.RightToLeft = RightToLeft.Yes;
radToggleSwitch1.ToggleSwitchElement.Thumb.MinSize = new Size(13, 13);
radToggleSwitch1.ToggleSwitchElement.Thumb.Margin = new Padding(2);

To reproduce:
- Just add a split button to a form and set its theme to Office2010Silver.

Workaround:
radSplitButton1.DropDownButtonElement.ActionButton.Margin = new Padding(0, 0, -1, 0);

See attached.

Workaround:

radCheckBox1.ButtonElement.MouseEnter += CheckMarkPrimitive_MouseEnter;
radCheckBox1.ButtonElement.MouseLeave += CheckMarkPrimitive_MouseLeave;


private void CheckMarkPrimitive_MouseLeave(object sender, EventArgs e)
{

    radCheckBox1.ButtonElement.CheckMarkPrimitive.Fill.ResetValue(BorderPrimitive.BackColorProperty, ValueResetFlags.Local);
}

private void CheckMarkPrimitive_MouseEnter(object sender, EventArgs e)
{

    radCheckBox1.ButtonElement.CheckMarkPrimitive.Fill.BackColor = Color.Red;
}

How to reproduce: check the attached video

Workaround: 
public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    public RadForm1()
    {
        InitializeComponent();
        
        ThemeResolutionService.AllowAnimations = false;
        this.radToggleSwitch1.AllowAnimation = false;
    }
}

To reproduce:
- Set the shape at design time with the shape editor.
- Start the application or restart the designer.

Workaround:
Set the shape in code behind.

Please look at the attached screenshot.

Workaround:
RadRadiomark radioMark = this.radRadioButton1.ButtonElement.Children[1].Children.Last() as RadRadiomark;
radioMark.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;

To reproduce:
public class RadForm1
{
	private RadButtonElement btnHelp;

	private void RadForm1_Load(object sender, EventArgs e)
	{
		btnHelp = new RadImageButtonElement();
		btnHelp.AutoSize = true;
		btnHelp.ShowBorder = false;
		btnHelp.Image = My.Resources.Resources.help;
		btnHelp.DisplayStyle = DisplayStyle.Image;
		btnHelp.ButtonFillElement.Visibility = ElementVisibility.Hidden;
		btnHelp.ClickMode = ClickMode.Release;
		btnHelp.Click += btnHelp_Click;

		this.FormElement.TitleBar.SystemButtons.Children.Insert(0, btnHelp);
	}

	private void btnHelp_Click(object sender, EventArgs e)
	{
		Process.Start("http://www.telerik.com");
	}
	public RadForm1()
	{
		Load += RadForm1_Load;
	}

}

Workaround:
Use the Press ClikMode.

To reproduce:
1. Drag a RadButton from the ToolBox and drop it onto the form. 
2. Drag a TelerikMetroBlue theme from the ToolBox and drop it onto the form. 
3. Change the button's theme at design time. You will notice the strange border around the control.
Please refer to the attached gif file.

However, when you save the form and reopen it the button looks ok.