You can use the filetypes config within lspconfig.lsp_name.setup({...}) to override the default filetypes on which a given LSP runs on.
For example, in my case I wanted the css_variables LSP to run on vue files, which it doesn’t by default.
This can be done by overriding the filetypes option on the setup function call:
require("lspconfig").css_variables.setup({
filetypes = {
"css",
"scss",
"less",
"vue",
},
})
Other options other than filetypes can also be overriden.
A full list of LSPs and their respective options supported by lspconfig can be found by running :help lspconfig-all, which is really useful for finding and overriding their default settings.