Use different units for line height to make it scale automatically with — or be independent of — font size.
To set the default line height for the entire site you’ll need to set the line height on the Body (All Pages) tag. The style will cascade down to all text in the site, unless there are more specific styles set on an element.
Changing the line height on a text element will only affect its text content. It overrides the styles that are coming from the element’s tag or the Body (All Pages) tag.