See no option for "Install new themes"

[mrmystery12]

I just installed a theme in the software center or "discover" yet I dont know how to apply it.

I never see a "Install new theme" in the settings

I'm using Lubuntu 18.10

[Dennis N]

After installing from software center, you have to select it from the Appearance dialog:

Menu > Preferences > LXQt Settings > Appearance
To Select theme used by applications:
Select Widget Style in the side bar, then use the drop-down menus beside "GTK 2 Theme" and "GTK 3 Theme" to select the theme.

Also in the sidebar:
LXQt theme affects elements like the panel and menus;
Icons, Font and Cursor are self-explanatory.