If you can't upload themes, images, etc. in WordPress, it's mostly an error related to the upload size limit or an error due to the file type.
If an error due to the upload size limit is displayed
It is necessary to change the upload size of PHP.
For details on how to change, please refer to Change PHP upload size (upload_max_filesize).
If an error due to the file format is displayed
It is reliable and safe to change the file to a format that can be uploaded.
(e.g. change SVG image to PNG)
You can allow uploading by using plugins or customizing WordPress, but please note that individual customizations and plugins are not supported.