Please use the following in your Theme Options -> Custom CSS
This will remove the dotted line.
border-bottom: medium none !important;
Regarding your question 2,
It’s saved using this function.
Our support does not include sharing programming knowledge.
You can hire a developer to explain to you in more details.
Regarding question 3,
Sorry, we do not have a documentation on custom fields, because we do not support customization.
We only have some user documentation.