Where to edit the registration process pages for a WPMU website ?

I'm looking to edit various things on the registration page (wp-signup.php) of my website, and the following pages (the blog creation page and the "go check your email" page).

I'm trying to do 2 things :
#1. Changing the CSS of that page so it looks better
#2. Change some bad wording of that page translation

For #1 (the CSS), some things I want to change seem to be directly written on the .php file and not in a separate CSS file (according to Firebug), which is weird for Wordpress.

For example, I want to add more space between the label fields of the page : http://www.envalais.ch/agva16/wp-signup.php

The code is :

.mu_register label {
font-weight: 700;
font-size: 15px;
display: block;
margin: 10px 0;

and I want to change "margin: 10px 0;" to : "margin: 30px 0 0 0;"

But I can't find where that code is, and how to overwite it in a "update-proof-friendly-way".

I tried doing it on the style.css file of a child theme of my theme (Buddy Social) but it doesn't work.

Any idea of how I can change the styling of these signup pages in an update proof fashion ?

For #2 (The translation), I'm trying to change some text, for example the "Donnez-moi un site!". So I checked in the French .po file of the bp-social theme without any success. No success to find that sentence either in the main fr_FR.po file of Wordpress...

Do you know which .po file à should edit to be able to change some texts of the wp-signup.php pages (all 3 pages of the registration process) ?

Thanks a lot for your help on these 2 points.