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Is it possible to change black color on background? My eyes get tired from reading because on black background color 
In your General Settings there's a "Forum Skin" preference, but I don't think it does anything. No matter what I change it to, it sticks to "Use Form Default".
That being said, it's probably quite easy to override the site's stylesheets with your own. It will take a bit of hackery, and the details will depend on the browser you use; DuckDuckGo (or whatever) for "custom style sheet MyFavBrowser", if you use a browser called MyFavBrowser.
Nope.
But if you are on mobile, you can use Tapatalk.
If you are on desktop, you can try some extensions to invert colors (I use Dark Reader to make everything dark.)
Or, use Vivaldi browser, it has built in filters: