Find a better dark mode background image for index page

[Cadair]

In #741 I added an inverted version of the light mode image, but there has to be a nice looking astronomy image which is actually dark we could use instead.

It doesn't want to be too busy and it wants to be biased to the top of the image like the light mode one is.

[pllim]

Hubble Ultra Deep Field is always a classic. Unless you want to wait for Roman First Look image? 😉

[hamogu]

Thanks to Olbers we know how profound it is that it's dark at night. So finding a dark image shouldn't be a problem...

[Cadair]

I think the deep field is probably not the best because it's to "spotty" with areas of very low and high intensity which can make reading text on top hard. You need something diffuse and nebula'y and it might need editing before use.

[pllim]

Is this diffuse enough? https://www.esa.int/Science_Exploration/Space_Science/Webb/Webb_s_wide-angle_view_of_the_Orion_Nebula_is_released_in_ESASky