This is likely because you have not ticked the Fill transparent area box.
However, since you make a selection to define the shape being filled, you should really be using the "Fill whole selection" instead of Fill similar colors. This of course fills the full circle, but if you move that layer under the layer with the house, you get the result you ...
-b on the command line
E.g. to get a white background instead of the now default transparent:
inkscape -b FFFFFF -h 1024 input.svg -o output.png
Tested on Inkscape 1.0.1, Ubuntu 20.10.
In Gimp, for a single layer (so without flattening the image)
Start the bucket-fill tool
Set to Behind mode
Set to BG color fill (or use Alt-click in next)
Bucket fill the whole layer (without using a selection)
This will "fill the holes" exactly as needed (especially the semi-transparent pixels on the edges).
try using Batch Image Background Remover and Changer on Android. This app perfectly suits your needs as it is best for working on multiple images at once and allows adding new backgrounds to multiple images and saving them all at the same time
You can download it from here > https://play.google.com/store/apps/details?id=com.enscyd.batchbackgroundremover
I don't have anything against the colors you have chosen. The iceberg being all white is actually quite fitting for a diagram of this kind in my opinion. Often "less is more". Nonetheless, I've given it a go and made a more colorful version for your inspiration.
(I didn't have the fonts installed so you'll have to live with Arial.)
I haven't used ...