Updating Value of XML Tag in C#

I am using a XML file as the main configuration (config.xml) source in my C# project, and therefore I need to be able to read data from and write data to this file. Included is the beginning of my XML which contains the tag I would like to update from within the app.

<?xml version="1.0" encoding="utf-8"?>
<root>
	<startupparams>
		<backColor>white</backColor>``` < -- Tag to update
		``` <foreColor>black</foreColor>
	</startupparams>
	<strings>
		<appTitle>TheCalc</appTitle>
		<appAuthor>Matthew Croft</appAuthor>
		<appGreeting>Welcome</appGreeting>

The problem is that I am not too familiar with C# and XML integration, and writing (or more accurately updating) the tags to the file is the process that is causing me issue as I am not able to successfully do it. What I tried is included below.

Console.Write("Background Color: ");
string background = Console.ReadLine()!;
Console.WriteLine(background);
Console.WriteLine();
Console.Write("Foreground Color: ");
string foreground = Console.ReadLine()!;
XmlNodeList backColors = config.GetElementsByTagName("backColor");
for (int i = 0; i < backColors.Count; i++)
{
    backColors[i].InnerText = background;
}

Keep in mind that I have already opened the XML by using the following.

XmlDocument config = new XmlDocument();
string configFilePath = @"C:\Users\mcroftdev\Documents\TheCalc\config.xml";
config.Load(configFilePath);

Any suggestions you can provide would be greatly appreciated. Thank you in advance!

The commenting in your xml made it invalid. But you may have done that just for documentation in your post. For testing I created this file in Notepad++. Notepad++ can tell you if your XML file contains errors:

This is the code I used. The primary change was to add a Save at the end. With a proper XML file and adding the Save the code works as intended.

XmlDocument config = new XmlDocument();
string configFilePath = @"d:\test.xml";
config.Load(configFilePath);
XmlNodeList backColors = config.GetElementsByTagName("backColor");
for (int i = 0; i < backColors.Count; i++)
{
  backColors[i].InnerText = "new background";
}
config.Save(configFilePath);