Besides, the player's ability to upgrade (or ruin) the visual style is exactly what I asked for. If they have a theme they like, they should be able to dump it into the main theme directory, and use if for all pak sets.
Thing is, this thread raises the issue that paksets should be able to provide their own themes and use them if not otherwise specified. Your post does not contradict that, but the way it's written suggests you are fine with the current behavior. It might help to state how you feel about pakset-dependent themes, because it could be we all agree on how it should be done, but by talking about different aspects we can't come to an agreement.
For me, as I said, two things would make sense:
1) CSS-like behaviour. Every browser has it's own CSS, which is usually overwritten by a websites CSS, but can again be altered by a user-specified CSS. But since the latter is rare, usually you see websites as they are intended. Same could apply to Simutrans with a basic theme delivered with the game, themes delivered with paksets, and the option for the user to beat that with a theme he downloaded somewhere.
Even in that case, I'd prefer if the users more obvious option is to change the theme of the pakset currently in use rather than being forced to change the theme for every pakset, especially since some will come with several pakset-themes of their own to switch between.
2) Using styles. Define several styles, like standard, big, high contrast and big high contrast, and what they should include. Then, each pakset could not only provide one theme, but a whole family which fits the description. If a player uses a 4k-screen, it's obvious he will need a big theme. But not nessecarily the big theme the game provides, but rather any big theme. I'm not sure how much information about a players screen and style Simutrans can read, but it would be great if Simutrans already starts with big themes if the screen is at least 3000px high, or with high-contrast if the OS-Theme is high contrast.
Either way, I'm mostly against the current situation were the player has to download a pakset theme himself and has to set it up, while possibly not even being aware of that option.