How can you convert a WebM with transparency to MP4 with a background color using ffmpeg?
The native VP8/9 decoders don't support transparency, so a) you'll have to force a decoder, b) the way to change transparent pixels is to overlay the video onto the color.

ffmpeg -c:v libvpx -i 123.webm -f lavfi -i color=c=0xD9DADB,format=rgb24
       -filter_complex "[1][0]scale2ref[bg][vid];[bg][vid]overlay=format=rgb:shortest=1,setsar=1"
       -pix_fmt yuv420p -c:a copy 123.mp4