Hey everyone I'm clearly doing something wrong and I'm not getting this to work...

-I have downloaded the dark them .deb
-I am incepted
-I have the latest ver. of twitter (stock)
-when i try and install it, it says "Cant install, the application has already been installed from a certified source."
- I have tried to copy the dark theme deb into /usr/share/twitter via file browsers and through terminal. With a you dont have permission to do this messages popping up.

What am i doing wrong?