I faced the same problem and wasn't able to fix it with the provided solution by Shamsudheen TK. Ionic somehow added transparency to my icons even if the source icon did not have any transparency at all. In the end I was able to resolve it by:
Install imagemagick (MacOS):
brew install imagemagick
Remove alpha channel from all images in resource folder:
find ./resources/ -name "*.png" -exec convert "{}" -alpha off "{}" \;