To customize and change the main color of the application follow the steps bellow.
1-Go to Values folder > open Colors.xml file and you will see all the xml file which is responsible for the main color and the theme of the main application
Also the file includes secondary colors for links and buttons and etc ..
3-Go to Settings.cs class and change the colors bellow to your own colors
//Main Colors >>
//*********************************************************
public static string MainColor = "#a84849";
You can control more your theme colors and status bar and text colors and dividers and etc from here the style.xml file is the main theme code of your application