Why should I care?
Thanks to GPLv3 if someone wants to distribute this theme with or without changes he must give the same rights he got from me to the people he distributes the changes helping Free Software growth.
Does it mean I cannot sell changed/unchanged copies of this theme?
Of course not. You can sell copies of this software with or without modification as long as you grant your clients the rights of GPLv3 to edit and further distribute your copies.
Should I distribute the source code if it is modified to my users if I use this software as a service?
No, you can do it if you want and would be great you do so, but it is not enforced.
Keep it simple, stupid.
Pequenio appears to be simple in appearance. Is it really?
When forking Pequenio from Micro one of the thing I cared most about was keeping appearance while adding functionalities like pagination in articles but simplicity also improved, before the theme used only a small part of Bootstrap, a big CSS framework but it had to load it in every request if cache was misconfigured. I cared about this and did a major css rewrite so only the needed css is used making a good cache policy unneeded to have fast loading times.
This theme Pequenio which is a fork of Micro does not link any external resources by default so it is keeping your user base data private so surveillance capitalism companies can not break into their privacy.
Also this theme attemps to be as simple as possible so you (And your users) can say goodbay to long loading times.
Why is linking external resources so bad for my users privacy?
Since most servers and browsers are not configured to avoid this using cookies nothing stop companies owning CDNs to track almost all navigation from everyone.
You can stop this!
Check the themes you use so they don't link against external resources for a better internet.