Stylesheet

A custom story stylesheet in Twine will automatically be included in your Twine story and work pretty much the same because they will both be plain CSS. Do keep in mind though that each Twine Story Format has a different way of displaying HTML content and so your stylesheet may end up looking different in the EPUB version.

In order to make sure your stylesheet is compatible with the resulting EPUB you may want to follow some basic rules:

Shallow CSS selectors

Despite the subtle differences between Twine's Story Formats and Spiner, most basic elements that markdown produces will be the same. So if you stick to styling only basic HTML elements, and don't make your CSS selectors to specific your formatting is going to behave quite the same.

Think of your readers peepers!

Unless you like to make a statement, going easy on custom styling is generally the most effective regarding e-books. A lot of thought has gone into the default style of most e-readers, and often any custom options by your readers as well.

Going easy often results into a more consistent, reliable styling as far as Spiner is concerned. And your readers might appreciate it as well.