In some cases you won't even recognize Bootstrap. For example YC-company https://kippt.com/ uses Bootstrap, but wouldn't guess it without knowing it.
Anyway, I think Bootstrap default theme (or slightly modified, but recognizable) is alright for young products targeted to "normal" people who have no idea about Bootstrap. I think it matters more for developers and designers and products targeted for them should invest in creating custom theme.
Anyway, I think Bootstrap default theme (or slightly modified, but recognizable) is alright for young products targeted to "normal" people who have no idea about Bootstrap. I think it matters more for developers and designers and products targeted for them should invest in creating custom theme.