Custom themes are not displayed after export if only .md file is open

[taunnol]

How to reproduce:

1. Open your .md file with presentation, not a whole project folder
2. Add a custom theme, specify the relative path to it in the settings
3. Theme will be displayed in preview
4. Export in any format
5. The exported presentation has a default theme, no errors appear in the console

This is very confusing, since the theme is visible in the preview, and the pictures (also relative) work great.