Weird horizontal lines and tweaks

https://susannayee.net/

https://thethemefoundry.com/blog/html-wordpress/

Strange horizontal lines under the header and under the very end of the sidebar. How would I remove them?

Also, the padding halfway down the page for entries works, while the first half is flush against the margin. Same goes for navigation, but it’s overlapping… how do I fix this?

Sorry, I am new at this!