Go to /view/front/themes/ , and make a copy of default master theme folder.

Rename it to whatever you want but keep it simple, single word names. I called mine mytheme . Not very creative, but for the purpose of this tutorial it will do.

Next, go over to /admin/configuration page, and set your theme as default one. Save changes.

Now open up theme folder /view/front/theme/mytheme/ and lets take a look at files.

Don't be scared by the number of files and folders, you probably will need to change only few of them.