A few more words on contract rights. This may or may not be spelled out in your contract, but the publisher has ALL editing and design rights.
Editing Rights--Your publisher can and will edit your text. Most of the time, these edits won't be huge. They don't have time to do that. If they want big edits, they'll tell you to do it. However, they will shorten sentences, cut paragraphs, and swap out words if they feel they need to do so. You may ask to have final approval on these edits. You may or may not get it.
Design Rights--This refers to cover design and page layout. You have no say in that. They choose the font, the layout, the margin widths, etc. They will hire their own artists to create the cover. You have no say in that either.
Most of the time, what the publisher does is going to increase the marketability of your book. Unless you are a professional graphic artist, they are going to have more experience in creating things that will attract the buyer. Trust them.
If they make a huge mistake and you can point out a legitimate marketing reason for changing the cover (for example, your main character is 45, but is portrayed as a 20 year old on the cover), they might listen to you and make changes. Or they might not. There's not a lot you can do about it.
If you have really strong opinions on editing and design, and this is a deal breaker for you, you may be able to have final approval to both added to your contract. Just know, that this may also be a deal breaker for the publisher too.