SyntaxHighlighter: How to remove the lime-colored question mark or at least change its color?

SyntaxHighlighter.defaults.toolbar = false;

In your shThemeDefault.css you can find the following code:

.syntaxhighlighter .toolbar {
    background: none repeat scroll 0 0 #6CE26C !important;
    border: medium none !important;
    color: white !important;
}

Above CSS difines to display green color '?' mark in you website.So, if you want to hide that then specify display : none in above code.If you need to change the background color you can specify you desired color in background property.

For version 3.0.x

SyntaxHighlighter.defaults['toolbar'] = false;