Hello there!
This is the way the theme was originally designed to behave. In order to fix it, you’ll need to do some customisation.
First of all, a quick suggestion: don’t edit the theme files directly, otherwise your changes will be overwritten whenever the theme is updated.
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.
Once that’s all ready, this CSS will show your sidebar on smaller screens:
@media (max-width: 1000px) {
.sidebar {
display: block;
width: 100%;
}
}