* 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
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