How to change to enhance interface of web site http://rikutoku.com/
See following article for explanation of this code: http://itsukara.hateblo.jp/entry/2017/10/30/223030
By downloading and changing html of rikutoku web page, "Red/Black" button is added for each problem. This button change the color of title of problem in the page. This is useful for marking difficult problem. Marks remain after closing browser and re-open browser.
As for change, see html-change.txt. It is very easy if you use gethtmlw (for downloading) and Speeeeed (for changing).
The following is a sample screen.