Why does Calibre Viewer mangle graphics background colors depending on theme?

stasiana · 10-03-2026, 10:59 PM

I have a sample epub which SVG images containing math. Sometimes they display correctly, and sometimes their background colors are mangled. It depends on the theme I choose in the viewer.

For example:

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It seems to depend on what the theme background is. What is this behaviour?

The test file: test.epub

kovidgoyal · Yesterday, 01:50 AM

It doesnt mangle anything. A theme sets background colors, depending on what CSS is in your book, it can have background colors that conflict, override or are overridden by the theme background. You can use the override all colors in book setting for consistency, which will override every color defined in the books own CSS

stasiana · Yesterday, 03:18 PM

Quote:

Originally Posted by kovidgoyal

It doesnt mangle anything. A theme sets background colors, depending on what CSS is in your book, it can have background colors that conflict, override or are overridden by the theme background. You can use the override all colors in book setting for consistency, which will override every color defined in the books own CSS

No, you're wrong. That is not what is happening.

Create a theme with background #737373. Works.

Change to background #727272. Mangled.

Even the app UI changes in response.

Martinoptic · Yesterday, 03:31 PM

But doesn't the theme generally change the UI - that's surely what they do?
I don't think I clearly understand the issue....

As Kovid said

Quote:

Originally Posted by kovidgoyal

A theme sets background colors, depending on what CSS is in your book, it can have background colors that conflict, override or are overridden by the theme background.

kovidgoyal · Yesterday, 07:37 PM

Sorry I can see no mangling 72 and 73 look basically identical to me.

BetterRed · Yesterday, 08:11 PM

If I select Sepia Dark in Viewer Prefs->Colors->Scheme:

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I get what the OP gets:

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BR

BetterRed · Yesterday, 10:29 PM

But - Mr Intuit suggested I try on a fresh install, and this is what I got:

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So, I had a look at my Config, and decided to rename the "viewer_??????.json" files to "viewer_??????.json.save"

When I ran the viewer, I got the same result. The only config file recreated was "viewer-webengine.json", it was 682 bytes - the one I renamed to "*.json.save" is 30,804 bytes.

So, looks like the problem is caused by something in "viewer-webengine.json". Because I don't use Sepia dark I'm not going to investigate further, but I'm attaching a zip with two .json files, one of which exhibits the problem and one of which doesn't, I removed search and file history from the 30,804b file to create the 'bad' one in the zip.

BR

kovidgoyal · Today, 12:23 AM

I meant there is no mangling in the UI. As for the book itself, the explanation is what I originally posted. The book's CSS conflicts. It tries to be too clever by half and detect dark mode based on colors and apply CSS filters to transform images which conflicts with the CSS rules calibre sets for dark mode themes, again as I said in my original post (though in the other direction) turn off override all book colors and you wont have the issue.

And I guess I can make the override CSS in calibre a bit smarter to outsmart such books by explicitly setting the background color for images and other media elements to transparent so common filters have no effect.

10-03-2026, 10:59 PM	#1
stasiana Member Posts: 18 Karma: 10 Join Date: Nov 2023 Device: Kobo Elipsa 2E	Why does Calibre Viewer mangle graphics background colors depending on theme? I have a sample epub which SVG images containing math. Sometimes they display correctly, and sometimes their background colors are mangled. It depends on the theme I choose in the viewer. For example: It seems to depend on what the theme background is. What is this behaviour? The test file: test.epub

Yesterday, 08:11 PM	#6
BetterRed null operator (he/him) Posts: 22,861 Karma: 34252730 Join Date: Mar 2012 Location: Sydney Australia Device: none	If I select Sepia Dark in Viewer Prefs->Colors->Scheme: I get what the OP gets: BR

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Yesterday, 01:50 AM	#2
kovidgoyal creator of calibre Posts: 46,527 Karma: 29634066 Join Date: Oct 2006 Location: Mumbai, India Device: Various	It doesnt mangle anything. A theme sets background colors, depending on what CSS is in your book, it can have background colors that conflict, override or are overridden by the theme background. You can use the override all colors in book setting for consistency, which will override every color defined in the books own CSS

Yesterday, 07:37 PM	#5
kovidgoyal creator of calibre Posts: 46,527 Karma: 29634066 Join Date: Oct 2006 Location: Mumbai, India Device: Various	Sorry I can see no mangling 72 and 73 look basically identical to me.

Today, 12:23 AM	#8
kovidgoyal creator of calibre Posts: 46,527 Karma: 29634066 Join Date: Oct 2006 Location: Mumbai, India Device: Various	I meant there is no mangling in the UI. As for the book itself, the explanation is what I originally posted. The book's CSS conflicts. It tries to be too clever by half and detect dark mode based on colors and apply CSS filters to transform images which conflicts with the CSS rules calibre sets for dark mode themes, again as I said in my original post (though in the other direction) turn off override all book colors and you wont have the issue. And I guess I can make the override CSS in calibre a bit smarter to outsmart such books by explicitly setting the background color for images and other media elements to transparent so common filters have no effect.