If you create a slide design (theme) and you want to save it as a template (.potx file), simply do the following:
Open the presentation that contains the slide design (theme) that you want to save as a template.
File > Save As.
In the Save As dialog box, in the File name box, type a name for the template.
In the Save as type list, click PowerPoint Template, and then click Save.
Note: Themes are not the same thing as templates in PowerPoint. Themes are made up of cohesive colors, fonts, and effects such as, shadows or reflections - all arranged in a slide design. Templates include themes, but they also contain instructions in placeholders suggesting what to insert and what types of information the user of the template might want to include.