Hi. Well, this was "awaiting moderation" all day... the second question I solved it!

But I have another one. How can I remove the space over the top/menu bar on my site?

You can see here:

http://www.playadictos.net/foro/

I want to get rid of the space below the logo and over the forum menu. Please, help me.

By the way, I'm using GeneratePress.

Hi @maltrain,

First of all, thank you for using wpForo!

Can I use any theme for my blog (not my forum) and for using cross posting later?

Yes sure, the wpForo – Blog Cross Posting add-on is designed to synchronize website/blog content with forum content, the work of the plugin does not depend on your theme. 

 I installed the forum here (  http://www.playadictos.net/foro/  ) and it's not full width... I don't know how to change to full width. I read it's on "pages" but I don't have that option (see "atributos de pagina" or "page attributes")

This comes from your current active theme. I suggest you use this CSS code, it'll solve the issue. Please add the code in Dashboard > Forums > Settings > Styles > "Custom CSS code" textarea, save it, delete all caches and check again. 

.container.grid-container {
   width: 100% !important;
}

body .grid-container {
   max-width: 100% !important;
}

But I have another one. How can I remove the space over the top/menu bar on my site?

Please add this CSS code in "Custom CSS code" textarea too, it'll solve the issue: 

.separate-containers .inside-article, .separate-containers .comments-area, .separate-containers .page-header, .separate-containers .paging-navigation, .one-container .site-content, .inside-page-header {
   padding: 0px !important;
}
#wpforo #wpforo-wrap {
   padding-top: 0;
}

Thanks for the reply!!!