There are a few ways to add CSS. If you want the styles applied across your whole site, you should write your styles in a stylesheet (file ending .css) and add it to your page template(s) or in your settings (Settings > Website > Pages > CSS & Stylesheets).
If you want your styles applied to one page only and you have a css file, you can add it to a page in the settings or, if you don't have a stylesheet, you can write the css in the advanced settings of a page in header or footer HTML like
I assume you want that copy left-aligned so it's inline with the heading? Whoever wrote the styling for this has written the left padding with "!important" on the id, which is difficult to override with one CSS rule, the easiest method is probably doing it with left positioning. Copy paste this into the header HTML section in the advanced settings of the page.