User style should be disabled in HTML mode

[laurent22]

https://discourse.joplinapp.org/t/best-way-to-import-stackoverflow-stackexchange-askubuntu-to-joplin-notebook/10746/2

[laurent22]

Note: this is because a web page by default would have a white background and dark font. But when the dark theme is applied on top of this, it gives a light font and dark background, which breaks rendering. The solution would be to force the light theme whenever rendering a note in HTML mode.