Godot how to change font size in RichTextLabel

RichTextLabel-font-size.cs

// adding override Font for default Theme
var fontForExplanation = new DynamicFont();
fontForExplanation.FontData = (Godot.DynamicFontData) GD.Load("res://Fonts/Xolonium-Regular.ttf");
fontForExplanation.Size = 24;
fontForExplanation.UseFilter = true;
fontForExplanation.UseMipmaps = true;
rtlExample.AddFontOverride("normal_font",fontForExplanation);

// or simpler way
var fontForExplanation = rtlExample.GetFont("normal_font","");
(fontForExplanation as DynamicFont).Size = 24; 

Theme resources change the Control's appearance. If you change the Theme on a Control node, it affects all of its children. To override some of the theme's parameters, call one of the add_*_override methods, like AddFontOverride(String, Font). You can override the theme with the inspector.

Note: Theme items are not Object properties. This means you can't access their values using Get(String) and Set(String, Object). Instead, use GetColor(String, String), GetConstant(String, String), GetFont(String, String), GetIcon(String, String), GetStylebox(String, String), and the add_*_override methods provided by this class.