The core plugin CSS file is found in the plugin’s folder (like all plugins):
I’m guessing you are using one of the core templates that come with Connections, yes? They do a have a few inline styles. The templates can be found in this folder:
Within that folder there are many other folders, each one for an individual template. Which sub-folder you would look into would depend on which template you are using.
Since you are using a local install, you’ll have to use the browser devtools to work out where and if there is a CSS conflict.
An easy way to see if it is a theme or another plugin is to temporarily switch to the default WP theme and deactivate all other plugins.
One last thing, if you deactivated the core CSS on the Connections : Settings admin page under the Advanced tab, then all core CSS has not being loaded, the only style would then be coming from the theme and any inline styles that might exist you the template you are using. This option is only for users who wish to code all their own styles. So I recommend leaving this active.
I hope this helps.