[DiapDealer]

Fairly self-contained Python code to make your PyQt5-based GUI plugin match Sigil's light/dark theme. It should be compatible with any version of Sigil that supports PyQt5 plugins. The dark theme just won't appear unless you're using Sigil 1.1.0 or higher.

def dark_palette(bk, app):
    supports_theming = (bk.launcher_version() >= 20200117)
    if not supports_theming:
        return
    if bk.colorMode() != "dark":
        return
    try:
        from PyQt5.QtCore import Qt
        from PyQt5.QtGui import QColor, QPalette
        from PyQt5.QtWidgets import QStyleFactory
    except ImportError:
        return

    p = QPalette()
    sigil_colors = bk.color
    dark_color = QColor(sigil_colors("Window"))
    disabled_color = QColor(127,127,127)
    dark_link_color = QColor(108, 180, 238)
    text_color = QColor(sigil_colors("Text"))
    p.setColor(p.Window, dark_color)
    p.setColor(p.WindowText, text_color)
    p.setColor(p.Base, QColor(sigil_colors("Base")))
    p.setColor(p.AlternateBase, dark_color)
    p.setColor(p.ToolTipBase, dark_color)
    p.setColor(p.ToolTipText, text_color)
    p.setColor(p.Text, text_color)
    p.setColor(p.Disabled, p.Text, disabled_color)
    p.setColor(p.Button, dark_color)
    p.setColor(p.ButtonText, text_color)
    p.setColor(p.Disabled, p.ButtonText, disabled_color)
    p.setColor(p.BrightText, Qt.red)
    p.setColor(p.Link, dark_link_color)
    p.setColor(p.Highlight, QColor(sigil_colors("Highlight")))
    p.setColor(p.HighlightedText, QColor(sigil_colors("HighlightedText")))
    p.setColor(p.Disabled, p.HighlightedText, disabled_color)

    app.setStyle(QStyleFactory.create("Fusion"))
    app.setPalette(p)

Then after you initialize your QApplication and before you show/exec it:

app = QApplication(sys.argv)

Add the following function call that takes the BookContainer object and QApplication object as parameters:

dark_palette(bk, app)

That's about it. Building your PyQt5 Application Widgets is up to you.

I may add some platform-specific tweaks and bug workarounds from time to time. I know for a fact there's some Mac color issues.