To make your own theme (e.g. a theme called "abc"), the layout of the files are:

styles/abc.css   (As soon as this is created, you can pick theme from tiki-admin.php?page=look)
styles/abc/*    (This is where you store all images and other assets related to your theme)
templates/styles/abc/*.tpl   (This is where you deploy .tpl files which override the ones at templates/*.tpl)

More details at:
https://themes.tiki.org/How+To+Add+a+New+Theme
