Each folder within a project can be assigned its own style irrespective of the style selected for the album as a whole. This is done by selecting a folder and entering 'Edit mode', see image right. If the Sprint panel is not open click on the small triangle next to 'Sprint' to see the setting. Click on the popup menu and select a style for the selected folder.
If different styles are only required for folders, and their sub-folders, at the root level then with this setting made, goto Sprint's General Settings and select 'Sub-folders inherit style of the root folder.'
Once the settings have been applied for all folders that you want to have a style, other than that selected for the album, just remake the album.
To override the album's theme image click on the folder icon and navigate to your theme image file for the currently selected folder. To remove the theme image either select a new one or simply highlight the text showing the path to the file and delete it.
Theme images can be selected from outside of the project, they do not have to be part of the album. A back up copy of the image is added to the project's 'res' folder, it will only be used if the original is missing at the time of album build - useful for images held on remote drives.