Installation of own *.sty files in TeXstudio?

Question

How do I have to include my own *.sty files in TeXstudio (with default setting, e.g. with PdfLaTeX)?

According to the log file TeXstudio uses TeX Live 2019 .

The *.sty files are organised in folders with separate subfolders e.g.

/beamer/base/themes/color/  
/beamer/base/themes/font/  
/beamer/base/themes/theme/  (the main *sty file ist located here, it include than e.g. corresponding color)  
/briefe/ ……  and so on

kpsewhich –show-path ls-R shows the following path:

/usr/local/share/texmf
/usr/share/texmf
/usr/share/texlive/texmf-dist
/var/lib/texmf

I have already tried to copy the *.sty folder into the first three paths as root and afterwards I executed sudo texhash.

Additionally I have created the subfolder /home/ich/texmf/tex/latex and tried to copy the sty-folder there, too, and executed texhash.

whereis texlive shows /usr/share/texlive.

kpsewhich texmf.cnf shows /etc/texmf/web2c/texmf.cnf. A comment inside it explains that /etc/texmf/texmf.d should be modified or own *.cnf should be used and then update-texmf should be invoked.

kpsewhich -var-value=TEXMFHOME shows ~/texmf.

kpsewhich -var-value=TEXMFLOCAL shows /usr/local/share/texmf.

I am using TeXstudio V 2.12.16 (Flatpak) on Linux Mint 19.3 Cinnamon.