Customize Timeline

Hi everybody,
I have been working with H5P for some time and I like it, it's a great tool for interactive HTML5 content. Now I would like to customize the visual appearance of the "Timeline". To adapt the images to our corporate design, I want to change the border of the images and give them round corners via the CSS property "border-radius". My idea is to use a CSS code like this:

#img-corner { border-radius: 25px; border: 2px solid #73AD21; }

I think the he easiest way would be to add this CSS-Code as an external style sheet, but directly via the editor this is not possible. At the moment we are editing the content with Lumi and integrate the .h5p file in Moodle.
As an alternative it would also be an option to get rid of the CSS-border completely and than we would integrate a graphic border direct in the image. 
Do you have any suggestions?

Thank you in advance
Chris