What's New figures background

[wpotrzebowski]

Not critical for this release but it would be good to standarize on the background of images we use for "What's new". It now uses Dark Theme and before was White.

[jellybean2004]

Sorry for this oversight! Since only a light background has been used previously, I will stick to that and update the images.

@sara-mokhtari, if it is not too much work, could you please also update the new models pic using a light background? No worries if this requires a lot of work, as the amount of dark bg seen in that image is trivial.