Divi and the Extra Theme share the same core functionality, namely the Divi Builder. This means that a subset of the Divi theme’s features are available on Extra – specifically the Divi drag-and-drop page builder and its modules. So Divi Booster features targeting the Divi Builder and its modules will work on Extra too.
Furthermore, as Divi and Extra have a similar code base, some Divi Booster features which operate on other areas of Divi may also work on Extra. However, at this stage I haven't done extensive testing to determine which these are.
In practical terms, this means the settings in Divi Booster under “Divi Builder” and “Modules” will work. A few others will too, such as the “Developer Tools”, and yet others may partially work, such as the “CSS Manager”. Of the remainder, I expect that some will work, but that quite a few won't work or won't work as expected.
Divi Booster Settings Page Location
When used with the Divi Theme, the Divi Booster settings page can be found by going to “Divi > Divi Booster” from the main WordPress menu. When used with Extra, however, you currently need to go to the plugins page and click the “Settings” link next to Divi Booster.
While I probably won't be making Divi Booster compatible with Extra (beyond the current limited compatibility), I have started providing tips specific to Extra on a new site, Extra Booster.
I may also consider releasing an "Extra Booster" plugin in the future, if there is enough interest. If you'd like to see this released and be notified when it is, please join my Extra Booster mailing list: