Why .title is connected to H3 and how can I change it?
H3 is a general heading class; H3.title is specific to the display of the title on each photo page.
Thus changes to H3 properties in CSS will affect all H3 headings (although I can't remember if there are other H3 elements in PBase that are not titles). Changes to .title properties will override the H3 properties, or add to them if they have not been defined in H3.
I'm not sure exactly what you are trying to change - it looks like the 0px border has done what you want?
Can I set the same design on my galleries? How can I do it?
You can use the same design by viewing the CSS from one my galleries, edit your own CSS, delete it and copy+paste my CSS to replace it.
Of course I can't guarantee that my CSS will always do exactly what you want. I would suggest not using my root gallery as there is some special code for the root - better to take the CSS from one of my other galleries.
If you want to preserve your current root design, you will need to have two stylesheets - one for your root containing the existing CSS, and one for the other galleries with the copy of my CSS.