Replace light-mode screenshot with dark-mode

[adiabatic]

Closes #3.

I tried making the screenshot pair responsive with picture/source. Unfortunately, GitHub rewrites image URLs for standard Markdown links (![…](…)) but not URLs in HTML.

[flexibeast]

Thanks for doing this! However, with regards to the PR's title, i'm not willing to simply replace a 'light'-mode screencap with a 'dark' one, given that i'm one of a number of people who find 'dark' themes difficult to read. My preference is to find some way of including both regardless of GitHub's limitations. So to that end, i've changed the guide to now read:

The resulting formatted output is visible in the following screencaps:

for dark-themed environments
for light-themed environments

using the screencap you provided. Does that work for you?

[adiabatic]

Sounds good to me. Here are a pair of light and dark screenshots if you're into matching pairs:

[flexibeast]

Sounds good to me.

Great, i'll close this PR then.

Here are a pair of light and dark screenshots if you're into matching pairs:

At this stage i won't use the screencap pair you provided, but might end up doing so at some point in the future. :-)