[Plugin: Category Posts Widget] Change Font size and colour

You need to find out your div id for that widget, (use Firebug, or source code)and style it in you child-theme style.css… It’s important to make something like this:
#widget div id {
}
.post-title {
custom styling code
}

p.s. plugin is getting style for titles from theme, so if you don’t name exact divid of the widget, your changes will affect all post titles.