Until now, we were following a similar concept of color schemes as what most OS
are doing. For instance, Freedesktop recently introduced a tri-state color
scheme of "Prefer Light", "Prefer Dark" and "Default", the latter being either
whatever the software prefers (e.g. we prefer either Dark or Gray for graphics
software usually) or what the system prefers. See #8675.
Until now, with GTK, we only had a boolean "prefer dark" setting through the
"gtk-application-prefer-dark-theme" settings. There is not even a "prefer
light".
Nevertheless for graphics application, there is clearly a third case (fourth if
we added a "follow system color preferences" which we don't implement for now):
gray mode and in particular middle gray. Having a middle gray UI is often
considered a necessity when working on colors in order to protect our perception
of color from being influenced by surrounding UI.
To fill this need, we were proposing a Default vs. a Gray theme in GIMP, but
this was a bit confusing and felt illogical, as discussed on IRC some time ago.
Also depending on whether you chose "prefer dark" or not for the gray theme,
this one was itself 2 themes, which made things odd and harder to work on.
Instead this commit:
- adds a color scheme concept in GIMP with 3 variants so far: light, gray and
dark. A possible fourth (future) variant might be to follow the system
preference (do all OS provide such a queriable option?).
- Our Gray theme is merged into Default (as the gray color scheme variant).
- Custom themes can add the following CSS files: gimp-light.css, gimp-gray.css,
gimp-dark.css which are the base file for their respective scheme. gimp.css is
still used as a fallback though it is not necessary (our own Default theme
does not provide a gimp.css anymore). Custom themes don't have to provide all
3 variants. A theme can just provide one or 2 variants if it only wants to
support 1 or 2 use cases.
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