1- Open “Theme Colors” folder.

2- Select all XML files in “Theme Colors” folder, Copy them.

3- Paste all XML files into your PowerPoint Theme colors folder.
*(Theme Colors folder path for each System you use)

01- For Windows XP:

C:\\Documents and Settings\[your user name]\Application Data\Microsoft\Templates\Document Themes\Theme Colors

02- For Windows vista:

C:\\Users\[your user name]\AppData\Roaming\Microsoft\Templates\Document Themes\Theme Colors

03- For Windows 7:

C:\Users\[your user name]\AppData\Roaming\Microsoft\Templates\Document Themes\Theme Colors

04- Windows 8, Windows 8.1:

C:\Users\[your user name]\AppData\Roaming\Microsoft\Templates\Document Themes\Theme Colors

OR Add them to: C:\Program Files\Microsoft Office\Document Themes 15\Theme Colors

Adding a custom background in PowerPoint is a fairly simple process. the easiest way to adding a background to your slides:

click on the slide you want to format the background on. Then right-click on the slide itself. The Format Background option will be the last option

If you’re not sure what your slide sizes are, you can go to Design > Slide Size  you can see in the below what is the slide sizes .

To learn how to save click here

To learn how to save click here

To learn how to change click here