Change how italics and bold are published.

I am currently using Graph Paper Press’ Berlin theme on my blog, by way of a child theme I created. I love the layout and functionality of the theme, and all the built in customizations, but what I do not like is:

a) the way the theme displays italicized text, with a yellow highlight behind it.
and
b) that when text is formatted bold in a post, it doesn’t actually show up as bold in the blog post.

I can add custom css to the child theme to make changes, but I just don’t know how or what that customization should look like for these two issues.

Any help or suggestions?