I accidentally deleted the default.css. I cannot get it back in the Project Files Pod, but I see it in my project on my laptop when I use Windows Explorer. I cannot create a new CSS because the CSS section of the Edit ribbon is now disabled.
Which version of Robohelp are you using? If RH2019, are you using Classic or New UI? Please always state the version (and Classic or New UI for RH2019) as the solution to problems can be very different depending on the version.
Because you used the term "pod" I'll assume Classic. Only stylesheets associated with at least one topic will display in the Project Manager Pod. So right-click on a topic and select a stylesheet in the Appearance tab - this should show all stylesheets anywhere in your project stucture even if they aren't associated with a topic. If it doesn't display though, you should just be able to browse to it using the icon.
To make sure the stylesheet is applied to all topics, you have a few options. Apply manually to all topics using the Topic List pod; apply to a master page and associate the master page with an output (ssl); associate the stylesheet with an output (ssl). The last two use the "Apply to all Topics" checkbox, but where you find the setting depends on the output you're using (most likely under Content).