How to change elementary OS Themes ? Is it possible to add new themes ?
This answer does cover your question about how to change themes: How can I add more themes on Elementary-Tweaks??
You can install new themes and themes' usually have their own instructions at their files.
You can see from elementary-tweaks the themes that is installed but it is not recommended since elementary-tweaks is no longer maintained.
Also you can use dconf-editor to change the theme (also developers warns that dconf-editor can show you some options that should not be messed in order to not break the system , dconf is a bit like the registry editor on Windows).
Open terminal and install if you don't have dconf-editor:
sudo apt-get install dconf-editor
Open dconf-editor at terminal:
CTRL + F and write gtk-theme, as changing the value of gtk-theme you can change your theme.
Also you can change your window theme with:
gsettings set org.gnome.desktop.wm.preferences theme "NameOfTheme"
Arc Theme have a readme file with a tutorial for install that theme