Many themes are free and MIT-licenced!
Bootswatch is a fabulous place to get a beautiful themes for Bootstrap. I have been using Bootswatch themes since Bootstrap 3. See it here.
When I have updated this website to use Bootstrap 4, Bootstrap is still in beta version. Right when Bootstrap 4 get to final release version, Bootswatch has released their themes for Bootstrap 4 too. It is really a great thing.
I have decided to update this website to have the theme selector too. So the theme can be changed easily. I add the theme selector button on the top right corner in the navbar. Please click on the button to see all theme options. Change it to your preferred theme to read this website. For example here, I choose slate theme.
There are more theme options. They all are beatiful and well-defined design. Here is the link to Bootswatch, https://bootswatch.com.
Thanks for reading!