I found an excellent topic on this matter, but the code mentioned there is outdated.
The post is in https://moodle.org/mod/forum/discuss.php?d=267662
And the author is Mary Evans
By now, we are using Moodle 3.6.2 and a theme, Fordson 3.6.1
I couldn't find demo code that works in Moodle 3.6, cause classes have changed since then. I can understand and use html and css, but it's kind of difficult to find a repository of examples made with responsive blocks like row, col , etc.
It´s all I need. Regards,