According to this page (https://developer.android.com/studio/write/theme-editor) in the official Android Developer Docs, accessing the Theme Editor can be done in two ways:
From an open styles XML file, such as styles.xml, click Open editor near the top-right of the file window. [Or,] From the Tools menu, choose Theme Editor.
However, I'm running Android Studio 3.3.2 stable on my Ubuntu 18.04 LTS operating system, and I am unable to find the Theme Editor in the Tools menu, nor does "Open editor" appear when I open styles.xml.
I can confirm that I experienced this issue with two of my applications. One app targeted Android 4.1 and the other app targeted Android 5.0, and I still could not find the Theme Editor.
I went to Find Action and "Theme Editor" was an option, but when I opened it, all I saw was a blank workspace editor. PLEASE HELP!