How can i add a custom script to a team site page? I have this new page UI designer that i don't get. How do i add a web part with my own css or js code? I can not find any solution to add my own code. Anyone got any ideas?
I'm not sure if there's a way to do it on the default team site pages, it is possible to open your team site with SharePoint Designer 2013 and add a Web Part page to the team site, which will have the ribbon that you are accustom to.
If you decide to add a web parts page but do not have access to some web parts, make sure your settings are configured properly as outlined here
Probably you must be using a Developer Template, where the Content Editor Web Part doesn't appear. You would have to create a new site collection using a Team Site template. Once done, you will be able to see all the settings you are used to, like adding a CEWP.
From the screenshot provided by you, it is a page used the Modern UI. So we cannot add the web part with CSS or JS code.
It is by design.
If you want to customize the modern team site, you could refer to the article below.
Customizing "modern" team sites.