In developer tools, go to the Network tab. Click the arrow (when you hover over it, it will say "Enable network traffic capturing"). It will turn into a square to indicate that it is capturing network traffic. Refresh the page. Click type to sort by type and look for "text/css". Double click the file. Click "Response body". You will see the CSS contents there.
To manually add styles, go to the DOM Explorer. Click an element in your webpage and it should bring you to that part of the webpage. You can add styles to the elements in the DOM Explorer or in the left window, you will see "Inline style" (if there are no CSS classes) or you'll see "Inline style" as well as the CSS class and its details. You can click in there to make changes.