"General concept" image is not black theme friendly

[YarikTH]

Describe the bug There is an explanatory image in the main README.md. As a modern guy, I use a dark GitHub theme with a black background. On such a background black lines are invisible and even blocks look out of place. Your image has a transparent background and clearly optimized only for white background. So

Expected behavior "General concept" image looks fine on any background. Maybe via embedding the white background into image itself.