RE: I’ve tried to install Tile plus template as a plugin but wordpress says that folder already exists:
Sounds like you renamed the zip file because WP was preventing you from installing it over top of the core Connections plugin.
RE: How can I customize css?
You can use custom template CSS override files, see this tutorial. Or you can add teh CSS to the theme’s custom CSS area.
RE: How can I display all entries in a single page (without pagination)?
Use the Tile Plus enable_pagination shortcode option, example
[connections enable_pagination=false]. I do not suggest displaying more than 50 at a time or the page may load too slowly. This is not a Connections limitation, the same would occur if you were to try to display as many WP posts on a single page. You can attempt it with more if you wish. page loading my be satisfactory.
Hope that helps!