There are a number of settings in the TT widget admin panel to control the look and behavior. If you are looking to change something that isn't a setting option, you can potentially override the style by creating a <style> block above the widget code.
Here is an example below that will make the testimonial text italic:
If you are using Chrome, one way to find the style, div or class name that an effect the style is by inspecting the widget using the developer tools. Here is a link to some info on that tool: https://developers.google.com/web/tools/chrome-devtools/inspect-styles/