This is just a simple exercise for me to learn some Go.
This proxy will invert the colors for any image, but only if the image is actually requested through the app. So a web page like this will work:
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But not a web page that specifies a host in the img url, like this:
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Unfortunately most big well-know sites fall into the latter category and thus don't work. Try smaller sites like personal website or local businesses. Here are a few that I found to work decently: