If we create a new (empty) Impress presentation, the slides are white or clear background; and if we create a shape it will have a blueish color. Yet - if we insert a table, it's colored in grays and dark-gray. This does not seem like a reasonable default choice. Given the rest of the colors / lack thereof. The same table color scheme seems to be used by default regardless of the template used. This is even more inappropriate. I believe something might be done for this issue specifically without solving the more general problem of "theme" support in Impress.
Tables now have no background colour, so I guess this can be closed. Arch Linux 64-bit, X11 Version: 7.5.0.3 (X86_64) / LibreOffice Community Build ID: 50(Build:3) CPU threads: 8; OS: Linux 6.1; UI render: default; VCL: kf5 (cairo+xcb) Locale: fi-FI (fi_FI.UTF-8); UI: en-US 7.5.0-1 Calc: threaded