I am no expert, and I have not used the Kirby theme, but the style you want is a simple bottom border like this:
border-bottom: 4px solid #000000;
You will need to check your files to find what element is used to wrap your titles. After that, you just add the above rule to your css. Make sure to keep original copies so you can always revert back if your attempts do not deliver desirable results.
The site you refer to is using an h2 tag for the post titles and they are using inline css rather than a style sheet. Of course, I could be wrong completely in which case you should hope for a more skilled individual to answer you question.
I personally play with all my files to see what happens and that way I have a chance of learning a little more. It just so happens, that I have a styling problem with my title tags as well, just a bit different.
Have a nice day.
