Add theme and plot thickness options (#847)

* Add theme and plot thickness options

* Cleanup

* Fixed themes, and made it so the plot colors are updated when the theme is changed

* I think supporting themes beyond dark, light, and black is unecessary for the time being.

* recall theme from settings before instantiating plot

* Check for null setting

* committing to merge

* Fix

* Cleanup

* Cleanup

* Fix from comments. Thank you kamkie!

* Fixing last UI bugs

* Fix windowed plot not theming on relaunch, change theme, show plot

Aga.Controls/Tree/NodeControls/NodeCheckBox.cs

@@ -55,7 +55,11 @@ namespace Aga.Controls.Tree.NodeControls
 		{
 			Rectangle bounds = GetBounds(node, context);
 			CheckState state = GetCheckState(node);
-			if (Application.RenderWithVisualStyles)
+            if (TreeViewAdv.CustomCheckRenderFunc != null)
+            {
+                TreeViewAdv.CustomCheckRenderFunc(context.Graphics, bounds, state == CheckState.Checked);
+            }
+            else if (Application.RenderWithVisualStyles)
 			{
 				VisualStyleRenderer renderer;
 				int scaledX = node.Tree.GetScaledSize(ImageSize, false);