How to add CSS to ID that doesn't change?

I want to put a white box around a text block and button block, so that they are both inside that box. Normally, in Wordpress etc., I'd do this by applying a white background to the column in which that the text block and button reside.

I know that due to Squarespace's structure, applying CSS with the column's the yui won't work, because the yui changes with each page refresh. So I am able to put a white box around either the text or the button, at the block level, but not BOTH.

Is there any way to do what I'm trying to do?

@jodybethw you can share url? I can take a look.