Inspecting the your CSS code of theme-elements.css there is a wrong width: 100% that’s the cause of the issue.
We don’t recommend modify the core files of the template like theme.css and theme-elements.css. If you want make some change consider to use the file css/custom.css and overwrite the current code.
To fix the issue you can just remove the width: 100%:
If you don’t want remove the CSS you can just add the class w-75 to the input, like: