Jill,
What you would be better off doing, is creating a child theme for your theme, then if you get any updates, it wont impact on any changes you make.
If you dont want to do that, you could just put the CSS at the bottom of the sheet under a heading. But you would have to remember to copy it before you update your theme (which is why id recommend a child theme as you wont have to do that)
If you are happy without a child theme, copy and past all of the below to the bottom of your style sheet:
/* CUSTOM CSS - TO BE COPIED BEFORE UPDATING THEME */
hgroup {
text-align: center !important;
width: 100% !important;
}
#logo {
float: none !important;
padding-right: 0 !important;
}
You can then keep adding custom CSS to the very bottom under that heading so you know what CSS you have written that will need to be copied out.
As before though, I would strongly suggest you look at child themes if you dont have a custom CSS area. Or you can install this plugin which would make your life just as easy instead of creating a child theme, it allows custom CSS:
https://www.ads-software.com/plugins/simple-custom-css/
Any questions, do ask. But in my opinion, if you install the above plugin and copy and paste the CSS you wont have to worry when your theme updates
Thanks