Hi Dave,
if you have a file "templates/core/loginform.mustache" in your theme then all is fine. If you also want to override the "templates/login.mustache" you need a file "templates/theme_boost/login.mustache"
That's the way templates work in moodle. In general you have to place a template in a folder with the full component name and in it the "mustache" file you want to override.
The loginform is a core component so the path in your theme is "templates/core/loginform.mustache"
The login.mustache comes from boost so the path is "templates/theme_boost/login.mustache"
Moodle look for templates in the following order.
- Current theme
- Parent theme of the current theme
- Component
What is found first will be used.
If you want to modify the context data that goes into the login.mustache you can override the file "layout/login.php" in the way I described in the last post.
Best regards
Andreas