Adhere to the system colouration.

Rationale

(At least ⪆ 3.0.0rc0 of) Darktable looks utterly different to every application on my OS. Occasionally, I have my OS set to a high contrast theme which Darktable can't match, and even when I don't, the inconsistency makes utilising its GUI unintuitive.

The Desired Solution

In effect, add color-scheme: light dark to your text/CSS, with prefers-color-scheme: implicitly set to the current value of org.freedesktop.appearance.color-scheme (whether no-preference, prefer-dark, or prefer-light).

Considered Alternatives

In descendent order of desirability:

1. Utilise a GUI toolkit which automatically does so (like Qt). However, I presume that this is infeasible.

2. Manually apply the colouration provided by the OS/DE:

   1. On Windows, utilise learn.microsoft.com/en-us/uwp/api/windows.ui.viewmanagement.uisettings.getcolorvalue?view=winrt-26100. ^1

   2. On GNOME or KDE, use the values in the gtkrc (or better yet, on KDE, adhere to the current .color file's INI-formatted values).

Additional Context

1. I'm not whatsoever suggesting CSD like issues/13555#issue-1573410812 does. Adherence to the DE's appearance in any manner except colouration is outside the purview of this issue. I also happen to not be fond of non-SSDs.

2. The current status-quo has, occasionally, caused invisible elements. ^2

Start by locating darktable's GUI text/CSS and the entry points that select application colours. Compare the requested light, dark, and high-contrast behaviour with the referenced Windows, GNOME, KDE, and freedesktop appearance sources. Done means the interface follows the system colour scheme without addressing unrelated desktop-environment or CSD changes.