I used a different approach. I use per-user themes and icons for the normal user. These are at ~/.themes and ~/.icons. My methods involve tricking the root apps into thinking that they're using current user's local theme,but in fact uses another global theme.
Method 1 : Using a systemwide/global theme and icons for Root Apps
Step 1 : Create symlinks in /root/.themes and /root/.icons to the global theme and icon you wish to use for root apps. Make /root/.themes and /root/.icons folders if they don't exist already:
sudo mkdir /root/.themes
sudo mkdir /root/.icons
I use the default Ubuntu Theme(Ambiance) and Icons(ubuntu-mono-dark).Replace Ambiance and ubuntu-mono-dark with the theme and icons you wish to use, and run
the following commands:
sudo ln -s -t /root/.themes /usr/share/themes/Ambiance
sudo ln -s -t /root/.icons /usr/share/icons/ubuntu-mono-dark
But make sure that the theme and icon are available at /usr/share/themes and /usr/share/icons,or else Root Apps won't be able to access them and they'll look ugly.
Note: The previous answers involved copying the entire theme and icon folders to /root/.themes and /root/.icons. A similar approach is used in Method 2 of this answer. But,i prefer symlinks as they avoid data duplication and gets the job done.
Step 2 : Rename the theme and icon symlinks in /root/.themes and /root/.icons to exactly the same name as the one from ~/.themes and ~/.icons that you're currently using. As i'm using Delorean Dark theme and Faenza-Darkest icons,the commands in my case would be:
sudo mv /root/.themes/Ambiance '/root/.themes/Delorean Dark'
sudo mv /root/.icons/ubuntu-mono-dark '/root/.icons/Faenza-Darkest'
Replace Delorean Dark and Faenza-Darkest with the names of the themes you use.
Shortcut Command: You may perform Step 1 and 2 in 1 step
sudo ln -s /usr/share/themes/Ambiance '/root/.themes/Delorean Dark'
sudo ln -s /usr/share/icons/ubuntu-mono-dark '/root/.icons/Faenza-Darkest'
Method 2 : Theme and Icons exclusively for Root Apps
If you wish to make Root App themes and icons not accessible to normal users,don't place them in /usr/share/themes and /usr/share/icons. Instead, place them in /root/.themes and /root/.icons. Then rename them to the same name as your current theme and icons, similar to Step 2 of Method 1.