Typo + improved instructions for Debian

Hi,

I found a typo in the instructions.

I also proposed an alternative method of installing dependencies which has the main advantage of marking them as automatically installed (not manually installed).

I do not know the equivalent procedure for non-Debian systems.

More details here: https://askubuntu.com/questions/101651/how-to-install-themes-with-gnome-tweak-tool/1128098#1128098

These suggestions also apply to the installation instructions on linux-apps.com: https://www.linux-apps.com/p/1136805/