My current workaround to that, on Windows, is a
StrokesPlus mouse gesture (Up, over the book cover in Cover Grid), which opens the title in a third party app called Sumatra.
Before that, I had tried setting Sumatra as default viewer, and a secondary StrokesPlus mouse gesture to open internal viewer, but this didn't go well because I do frequent FTS searches, and they depend on internal viewer as default for the main formats.
A second solution I'd tried was setting the Calibre 3.48 viewer Portable as default, which is fast at opening... but doesn't have a dark theme.