The easiest way is to simply hide the sidebar toggle.
#menu-toggle {
display: none;
}
Don’t edit the theme files directly, otherwise your changes will be overwritten every time the theme is updated to the latest version
An easy way to add custom CSS is to install the Jetpack plugin and activate the Custom CSS module. You’ll then add your custom CSS in the new stylesheet editor that’ll appear in your dashboard, under Appearance > Edit CSS.
As alternatives, you could either install a standalone custom CSS plugin, or create a child theme.