Here I summarize what done for the title.
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OS = Ubuntu, with texlive and latex installed.
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(Get XXXX.sty files from e.g. webpages.)
ls ~/Download/ ==> "XXXX.sty" exists.
cd /usr/local/share/texmf/tex/latex/
sudo mkdir styles
(Input your password.)
cd ./styles/
sudo cp ~/Download/*.sty ./
sudo mktexlsr
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