You've found a theme that almost fits. Just a bit of CSS to tweak, maybe move a widget. The solution is not to edit the theme files directly. That works until the theme gets updated, and then your changes are gone.
What a child theme does
It inherits everything from the parent theme: templates, styles, functions. But you can override individual files. WordPress always checks the child theme folder first. If it finds a file there, that's what gets used. Otherwise it falls back to the parent theme.
This means the parent theme can be updated without losing your customizations. Simple principle. Enormously practical.
Create the folder
In wp-content/themes/, create a new folder. The naming convention is parenttheme-child. You need two files: style.css with a comment header pointing to the parent theme, and functions.php that enqueues the parent's styles.
Important: load styles with wp_enqueue_style(). Not with @import. It's 2025.
Override a template
Want to change how single.php looks? Copy it from the parent theme to your child theme folder. Keep the same filename and folder structure. Make your changes in the copy. WordPress finds it automatically.
Version control with Git
Put the child theme folder in a Git repo. Makes it easy to track exactly what changed, roll back mistakes, and collaborate with other developers. The parent theme is already version-controlled by its developers; that's not your problem.
The whole process takes maybe ten minutes. Invest them. You'll thank yourself at the next theme update.