Is there a way to edit the master page without using SharePoint Designer? I tried to use the Oslo theme but it doesn't show on my site, i looked in the internet and using Desinger may be the only solution.
You can edit a master page with any text editor of your choice. For example: I always download the default master page (seattle.html or oslo.html) and do my stuff with notepad++. After that just rename the file so I don't overwrite the default file and upload it. After that just publish master page so it will be converted to .master-file.
You can also check if anything is right with your new master page by calling
You can download the file from the Page Layouts and Masterpage gallery. After you have obtained it from the website, you can edit it in any text-editor you like (notepad, VSCode, etc.). When you are done you can upload the file again and see your changes.
Same question for your reference: