Can you show us the 'original' .png icon you are using?
If it’s something like this:
You can remove the white background via Format > Remove Background.
Then, right click the image > Format Picture > Picture > Picture Corrections > Brightness/Contrast > Brightness > +100%.
Click the image below to see this process. Hopefully it suits your needs.
Note – I have a coloured background page so the white icon can be seen.