In other words, certain details are lacking as to what source were you copying from, how did you copy it into sigil (e.g. into book view? load the HTML from file view? something else?)
Once in Sigil, you should be able to examine the (X)HTML and CSS files (if any) and determine what is controlling the styling.
If there is no stylesheet, look in the HTML file(s) to see the <style> definitions in the <head> section.
With the book open in Sigil, you should be able to use the previewer (f10 in linux, ymmv) to see what styles are being applied. Fix those styles to suit you.
No, I can't think of a macro that will do this for you automatically
j/k, no offense. Just riffing off your usual posts.
Albert