How to keep EPUB diagrams readable in dark mode
Check the same file in light and dark themes on the reader you use. Do not assume that a theme setting changes images in the same way as body text.
On this page
Read labels in both themes
Switch between light and dark modes while looking at the same figure. Check thin lines, legend text, shaded regions and a caption. Body text may become comfortable while an illustration remains difficult because image colours do not necessarily follow reader text colours. Inspect the actual figure rather than the cover.
Distinguish an image from styled text
A label inside a picture behaves differently from a caption made of EPUB text. If the caption changes colour but the diagram does not, those elements need separate checks. Try the theme you actually prefer on the device you use. A screenshot from another reader cannot establish that your own theme will handle transparency or contrast the same way.
Test image contrast before changing its background
Inspect the useful figure in your reader’s light and dark themes. If transparent areas make dark strokes disappear, compare the image resource with its source before editing. In a backed-up EPUB, a light background on that figure container may help; avoid forcing every image to the same treatment. Reopen the edited file in both themes and inspect small labels, legend colours and surrounding text. No new dark-mode conversion result is reported here.
These are targeted repair steps for your own file, not a newly executed before-and-after demonstration. Keep the unchanged original, an edit log and the exact reader version. Validate the saved copy and compare the full passage with the PDF before relying on the result.
Keep a usable alternate view
If dark mode makes an essential image unclear, inspect that passage in light mode or use the PDF. Do not alter data colours without understanding the legend. If you edit a reading copy, compare the revised illustration with the source and retain both. Comfortable prose is useful, but the chart must still communicate the same categories and values.
A worked reading example
A paragraph can be comfortable in dark mode while thin diagram lines become hard to see. Switch themes and inspect labels, legends and shaded regions separately.
Do not change the legend’s meaning
A chart may use colour to identify categories. Improving dark-mode contrast should not swap categories, erase distinctions or turn a shaded region into a different apparent result. Compare the legend and labels with the source after any alteration.
If the app offers an image-specific viewing option, consult its own guidance and check the figure there. Otherwise, use light mode or the original PDF for the relevant passage. Keep notes about the theme and reader rather than treating a dark-mode screenshot as a universal EPUB property. The aim is both comfortable contrast and preserved information; one is not a substitute for the other.
Try a meaningful figure rather than a decorative photograph when judging dark mode. A photograph may remain attractive while a thin-line diagram loses essential labels. Keep the theme decision tied to the document you are reading and the information you need, rather than assuming one global preference is always comfortable.
Check the actual retained figure in both themes
The actual NASA sample EPUB retains labelled hardware drawings, but we have not verified that figure in a dark-theme reader. Open the actual file in your two themes and inspect fine strokes and labels. That task tests your reader presentation separately from retained pixels. EPUBTurn converts the source PDF; background repairs to an existing EPUB remain an editor task.
Open the sample in your reader’s light and dark themes and inspect diagram labels.
NASA source
D. Harold Shelton, Apollo experience report: Lunar module stabilization and control system, NASA, 1975. US government work; public use permitted. Original pages 3–8 extracted; sample selects excerpt pages 4–6.
Files used in this guide
Download the source files, conversion results or worksheets referenced here. Each label identifies the source document or the converter used.
EPUBTurn actual NASA sample: three selected pages