custom background colour with #id selector not working

I would like this page [ https://www.barbaramacias.com/paros2020 ] to have a different background colour than the rest of the site [ https://www.barbaramacias.com/ ] . Because the template only allows for one colour, I thought I could solve it by adding custom CSS. The html element has an id and a class. When I change the background colour in Chrome Inspector, it works but when I add the same line to the Custom CSS window in Squarespace, it does not work. I tried using these three versions of the CSS selector (just the id, id +class, tag+id+class) but none seem to work:
 

#yui_3_17_2_1_1578652083284_68 { background-color:#ffffff;}

#yui_3_17_2_1_1578652083284_68.Main.Main--page { background-color:#ffffff;}

main#yui_3_17_2_1_1578652083284_68.Main.Main--page { background-color:#ffffff;}

What am I missing? Thanks!

#yui will change if you reload the plage

so don't use #yui

Use Page ID. See how to find Page ID.

If you use Business Plan or higher, remove the id, and wrap css in style and add to Page Header

<style>

css here

</style>

thanks a lot, that worked!