Just wanted to share with you that in case you plan to upgrade your pre-v5x theme to match the latest theme structure, you may notice that some of the CSS and images have been moved to plugin ZIPs (as part of the v5x code separation core/plugin concept).

No worries though, luckily the internal S:P logic allows to have these images, for instance found under

/yoursite/path/to/sp-resources/forum-plugins/private-messaging/resources/images

to be also stored in:

/site/wp-content/sp-resources/forum-themes/mycustomtheme/images

Works like a charm!