It is called a (linked) 'stylesheet'
p{ your style stuff here }
What that says is apply that to ALL 'p' tags
Now this is real basic and you will probably want more, but get OUT of the habit of inline style="blahblah' coding.
You will end up with a nightmare if you want to change, whereas there is ONE place to make a change with a stylesheet.
Take a ebook you like and look at the stylesheet
click once on the tag for a paragraph, then right click: Go to link or style.
Sigil will normally take you right there