Add default brushes to controls to prevent rendering issues

[bhavanesh2001]

Description of change

This PR adds default brush initialization to several controls that previously lacked them. Without defined brushes or colors, some controls — such as the Slider — were not rendering at all.

If i define controls something like this

 <controls:Slider />
 <controls:ToggleSwitch />
 <controls:SegmentedControl>
   <controls:SegmentedControl.ItemsSource>
     <x:Array Type="{x:Type x:String}">
       <x:String>Option 1</x:String>
       <x:String>Option 2</x:String>
       <x:String>Option 3</x:String>
     </x:Array>
   </controls:SegmentedControl.ItemsSource>
 </controls:SegmentedControl>
 <controls:NumericUpDown  />
 <controls:LinearGauge Value="50"  />

The slider is not visible at all, and other controls have issues too..

After Fix

Fixes #64

[jsuarezruiz]

Could you create a static class where define the Controls Themes with colors etc?