If something keeps changing your intended css into useless code, this may be a situation where you would resort to simple text (eg. nano for mac or notepad for windows) Web design programs are double edged swords, most of the time the bells and whistles on these programs help make things easier, but sometimes they can make things more complicated. To custom style a button all you have to do is put your id or class selector name in the input tag and then enter the css for it. For example
CSS
#code_check_button { background-image: url(/*desired image url*/);
background-color: /*desired background color*/;
color: /*desired font color*/; }
HTML
<input id="code_check_button" type="submit" name="submit">
Just try it in notepad this time.