chore(README): fix up conditional logo rendering

[CompeyDev]

A really simple 1 liner change, which has been annoying me a bit lately.

This PR makes use of GitHub's provided theme URL fragments, instead of separate conditional media sources which was previously used, and didn't seem to apply correctly for me sometimes (see below). I was able to reproduce this without any userCSS themes as well.

Overall, I just think it makes more sense to offload things like this to GitHub themselves, instead of handling it on our own.

[TayouVR]

I don't think this is working right like this..

[lordofpipes]

The syntax already used in the README.md matches exactly how Github says the feature should be used:

https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#specifying-the-theme-an-image-is-shown-to

This feature used to be a Github beta feature but it has since rolled out to all users, so if it's not working, it's a bug with Github. So this should probably just be reported to Github since most people can't replicate the issue.

This article seems to frame #gh- as the old, clunky way to do it.

[lordofpipes]

Oh, I was able to replicate the issue by setting Theme mode to Sync with system, and then flipping the day theme to dark and the night theme to light. The issue doesn't seem to manifest when just setting a non-synced theme.

[CompeyDev]

Oh, I was able to replicate the issue by setting Theme mode to Sync with system, and then flipping the day theme to dark and the night theme to light. The issue doesn't seem to manifest when just setting a non-synced theme.

Hm, I have the dark dimmed theme set, which isn't synced with system and I seem to face it too. Odd.

[CompeyDev]

I don't think this is working right like this..

This is exactly how it functions, this is intended functionality. When on dark mode, any light mode images, when fetched, are returned as empty images and vice versa.

[CompeyDev]

After a reading of the article, I feel like the problems highlighted there should mainly apply to things to be published to external sites. Unless a GitHub scraper or something similar just extracts and displays the Prism launcher README, I don't think we should worry about that, tbh. Regardless, it should probably be a feature of such scrapers to maintain an environment similar to GitHub since they would possibly be rendering things configured for GitHub.