This issue is closely related to #2779919: Stable theme does not respect "visually hidden" field label display option; the following description is a minimally changed copy of the current issue summary there. It may also be informative to look at #2779743: Hidden/Visually hidden field labels don't work.
Site builders can set field labels to be hidden (not output at all) or visually hidden (text is available to screen readers) via admin/structure/types/manage//display. The Zymphonies theme does not add the proper class to the field label to achieve the "visually hidden" effect.
Steps to repro:
Standard install profile with some devel generated content
Navigate to admin/structure/types/manage/article/display
For the "Image" field, change the "Label" setting to "Visually hidden"
Enable and set as default the Zymphonies theme
Navigate to an article node
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Comment #2
zymphonies-dev commented