git clone https://git.eigenlab.org/sbiego/transmission-web-soft-theme.git
Copy the content of the theme (recursively and verbose to check output) in the transmission daemon root directory (usually: /usr/share/transmission/web/).
cp -rv transmission-web-soft-theme/web/* /usr/share/transmission/web/
Read the code in install.sh and if everything seems fine with your system run:
chmod +x install.sh
will install (overwriting just the current favicon) and let you choose to apply or not the theme.
See enabling/disabling section