I opened a related draft implementation separately, but the broader idea is simply a possible follow-up to this feature:
The current single-SVG approach with var(--primary), var(--secondary), and var(--tertiary) is still the clean/simple path for most sites, especially where the same SVG just needs to adapt its colours.
The use case I was exploring is the edge case where a dark-mode splash needs a genuinely different asset or optical treatment, rather than just recolouring the same SVG. For example, a logged-in dark header may work best with one transparent logo/background treatment, while the anonymous login/splash view may need a slightly different charcoal background or adjusted SVG for better contrast.
So the idea is not to replace the current variable-based approach, but to provide an escape hatch for sites where the dark splash artwork really does need to differ from the light/default splash artwork.
Feedback welcome on whether this belongs as a separate splash_screen_image_dark setting, or whether the existing single-SVG + colour-variable approach should remain the only supported route.