For example,
if your fontstyle is bold, then when you set italic, there are many people make mistakes,
and looses bold style while applying bold effect.
Solution:
//store the current font style
FontStyle fs = richTextBox1.Font.Style;
// now you can store all the style whichever you want
fs = fs | FontStyle.Bold|FontStyle.Italic;
//now you can use this fontstyle to apply style of text
content.
richTextBox1.Font = new Font("verdana",10.8f, fs);