When you create a new presentation, you add a new slide, then if you select the "Gradient" format as the background option, after you configure it with the colors you want you save it. When it's time to open the presentation again, you background colors have been set to black-white instead of the colors of your choice. So the problem is that whatever colors you set in this background format after you save the presentation and you open it again the colors are set to the default ones (black-white).
Thank you for reporting the bug. it seems you're using an old version of LibreOffice. Could you please try to reproduce it with the latest version of LibreOffice from https://www.libreoffice.org/download/libreoffice-fresh/ ? I have set the bug's status to 'NEEDINFO'. Please change it back to 'UNCONFIRMED' if the bug is still present in the latest version.
Works for me in Version: 6.4.0.0.alpha1+ Build ID: fa7f07d23de31e5f1660a643d83ca8a2e565f69c CPU threads: 4; OS: Linux 4.15; UI render: default; VCL: gtk3; Please test with newer version. Thank you
This situation was recently fixed as part of bug #125449 *** This bug has been marked as a duplicate of bug 125449 ***