I just updated my blog to WordPress 4.8, which contains a number of feature updates mostly to do with widgets. As always, I backed up my files and database first. I have automatic updates disabled so I can do this beforehand. The TwentyTwelve theme was updated but the only file that was changed was the style.css that contained rules related to the new widgets. I did not have to change my child theme since it loads the parent’s style.css automatically.
The only quirk I noticed was in the post editor I am using right now. The line-height did not match that of the theme and it bugged me. After inspecting the CSS, I discovered I needed to add the following to the my child theme’s editor-style.css to override some other styles:
line-height: 1.714285714 !important;
After this, I then had to clear my browser cache to actually see the changes. Other than this, everything seems to be working fine.