As far as I know, the only way to add a table currently is to use HTML table elements (<table>, <td>, etc.),
but they don’t appear to take on any styling (e.g., elements are aligned correctly in a table, but don’t show borders).
I created a new test blog and tested some custom CSS in my blog settings and it does apply if you create an HTML table within your blog post. I copied/pasted the example from this tutorial for a quick test: https://www.w3schools.com/html/tryit.asp?filename=tryhtml_table_intro, but I put everything in the <style> tags in my custom CSS in blog settings (and delete the opening and closing <html> and <head> tags)
A more elegant solution would be available in extended markdown, as discussed briefly here: New formatting syntaxes
Someone with a bit more HTML/CSS experience might be able to weigh in with a better answer