These days I mess around with files in /usr/share folder, like modify themes and other stuff... So, is it safe to change permissions of /usr/share folder so I don't have to open it as root every time I want to change something there? Or at least just /usr/share/themes folder?
If you are the only user on your system changing permission
/usr/share/themesshould be safe. But it's usually a better idea to use your home folder for the files you are working on. Just put your themes in$HOME/.themes/(create the folder if it doesn't exist), you can use them from there, too.