I installed a cool skin called "Pure Forums",. I see it on the backend, but don't see any way to make it default (both site wide and for my collection).
For site wide do you mean for all collections under a single install of b2evo.
Each time you create a new blog/collection there is an option to select a skin, after that you can use the back office to change each one, which is the then the default for all users unless you provide the option for other members to select their own skin.
EDIT You can force a skin in the [/conf/_basic_conf.php] by adding the variable $skin='manual'; for example
Oops! The above line is not right. The place to force a skin is really the index.php or a stub file for each collection. See https://b2evolution.net/man/stub-files
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For site wide do you mean for all collections under a single install of b2evo.
Each time you create a new blog/collection there is an option to select a skin, after that you can use the back office to change each one, which is the then the default for all users unless you provide the option for other members to select their own skin.
EDIT
You can force a skin in the [/conf/_basic_conf.php] by adding the variable $skin='manual'; for example
Oops! The above line is not right. The place to force a skin is really the index.php or a stub file for each collection. See https://b2evolution.net/man/stub-files