Now I see that you can add custom CSS as text by opening a page, selecting the "Page Design, Location..." gear icon, and then choosing "Design", and then "Theme > Customize" and scrolling to the bottom. Two things: one a question and another a request.

1) Can someone confirm if this applies sitewide to the theme, or just this page in question?

2) Could a better location for theme customization be considering for future versions of 5.7? i.e. not requiring going into a page's attributes to make these changes?

What you're describing was actually never the case in 5.6. Maybe you had a theme that pulled some magic that made this work?

If you add style customizations in the CSS section of the customizer, and then save them, you can choose whether they apply to the whole site or to just the page in question.

I was using Bootstrap in 5.6. Maybe that's why it worked? I will switch out to the base theme and see if that makes the difference.

Re: 5.7 CSS. What's weird is the first time I tried to do this I could have sworn it did not ask me to apply it sitewide or just the page in question. Now I see the prompt. Not sure how I missed it before... anyway, thanks for the reply.

No problem – I just ran through this and it seems to be working for me (applying it to one page vs. applying it to all)