How do I write my own CSS for a block?

Every block on your pages carries class names that do not change, so a rule you write today keeps working. .pspun-block matches any block. .pspun-block--media matches a picture, .pspun-block--prose a piece of writing, and so on: the pattern is .pspun-block-- followed by the block's name in lower case. A Container is .pspun-container, and the blocks inside one carry their own names as well.

Write the rules in your website's Code section, under Custom CSS. They load after the theme, so they win.