There seems to be a difference in widgets between main website pages and the forums. For example, look at the social login widget on these two pages:
https://www.urbanfiction.org/community/
It clearly looks better on the main page. You will also notice that the text is bigger in the widgets on the forum. I like that. How can harmony be acheived?
Font-sizes are the same. The only difference is underlines, which comes from your theme. For some reason it adds underline to all links. Using this CSS code you're remove the underlines from forum and widgets:
body.wpforo .post-content a {
text-decoration: none !important;
}
Using this one, you'll remove the underlines from widgets only:
body.wpforo .post-content .wpforo-right-sidebar a {
text-decoration: none !important;
}
But what about difference in text size and layout? It makes it smaller.
How it should be like on main page of site.
The wrappers are different, so the tree of CSS classes are different too, they can't be identical. The wpForo sidebar is an extra sidebar loaded in page content, it's not the a regular sidebar of your theme. If you want look them identical you should use the right sidebar of your theme instead of wpForo sidebar. Just remove all widgets from wpForo sidebar, then change forum page template from full width to 2 col with right sidebar.