Use can use ThemeKey module.
Enable ThemeKey module and its required sub modules, then go to the config page at admin/config/user-interface/themekey
. Here you will see may options for switching the theme.
As you required there is an option path:node_alias
so select this option and also set the value as you want like you say /gallery
and then select the theme you want to enable at this url.
For wildcard path use this setting as you required web/*
:
Property: drupal:path
Operator: =
Value: %wildcard
Property: drupal:path:wildcard
Wildcard: wildcard
Operator: ~
Value: /^web/.*
Like this image reference: