Updated EquationButton contrast fixed dark mode foreground color bugs (#1155)

* Fixed contrast between background and foreground colors in the equation button. Fixed the issue where the text in the equation text box is white when the background is white

* Adjust the foreground color algorithm

* moved the contrast method to utils so that it is resuable

* Moved brushes for the GetContrastColor method to the app.xaml resource dictionary

* Removed the change for the edit box colors, so it can be in a different PR

src/Calculator/App.xaml

@@ -293,6 +293,10 @@
             <x:Double x:Key="CalcOperatorCaptionSize">14</x:Double>
             <x:Double x:Key="CalcOperatorTextIconCaptionSize">16</x:Double>
 
+            <!-- White and black color brushes used for the Utils::GetContrastColor-->
+            <SolidColorBrush x:Key="WhiteBrush" Color="White"/>
+            <SolidColorBrush x:Key="BlackBrush" Color="Black"/>
+
             <!-- Base style for calc buttons -->
             <Style x:Key="CalcButtonStyle" TargetType="Controls:CalculatorButton">
                 <Setter Property="MinWidth" Value="24"/>