Open a presentation and apply all the customizations you want. Next, go the Design tab and click the down arrow in the Themes group. Then, select Save Current Theme.
To set the theme you've created as default theme, right-click on it and select Set as Default Theme. Now, every time you create a new presentation, it will be used by default.
To apply your theme to your old presentations, open one and access the Design tab to set your customized theme. Save the changes and so on. You may need to do some ajustements to the text or images before saving the presentation.