You’ll need to have a Gmail/Google account to use this option. Point your web browser to drive.google.com and click “Go to Google Drive.” Enter your Gmail username and password when prompted. Once your account has been authenticated, your Google Drive account will appear. Click the “New” button on the top left corner of the screen, then select “File Upload.” Navigate to your PowerPoint presentation and click “Open.”  Once the upload completes, a confirmation will appear at the bottom of the screen. Double-click the name of your PowerPoint file in that box to launch it in the viewer. When the preview of your presentation appears, click “Open with” and select “Google Slides.” It may take a few moments for all of the slide data to appear. Click a slide on the left side of the screen to change its background. If you want to change the background for all slides, you’ll be able to in a few moments. Open the “Slide” menu at the top of the screen and select “Change background.” You’ll see a previews of your selections as you browse through options. If you want the background of your slide to be a single solid color, click the box next to “Color” and select one from the palette. If you want the background to be transparent, click “Transparent” above the color palette. To make your background an image, click “Image.”  If your desired background image is on your computer, click “Upload,” then click “Choose an image to upload.” Navigate to the image location and click “Open,” then “Select.” To use an image from your Google account, click “Google Drive” and navigate to the location of your desired background image. You can search for the image name in the search box if you’re not sure where it’s located. Once you find it, double-click it to save the selection. If you decide you don’t like your background selection, click “Reset Theme.” To apply your new background choice to the slide you’ve selected, click “Done.” If you want to apply this background to every slide in your presentation, first click “Add to theme,” then “Done.”

TL;DR in Indonesian:
Buka Google Drive. Unggah presentasi PowerPoint. Pilih slide yang ingin dimodifikasi. Lihat opsi-opsi latar belakang slide. Pilih sebuah warna untuk latar belakang slide. Gunakan gambar sebagai latar belakang slide. Klik "Setel ulang ke tema" (Reset Theme) untuk membatalkan perubahan dan mengembalikan latar belakang sebelumnya. Simpan latar belakang.