right?

{
module : " calendar",
classes:"foo",
config: {
.
.
.
}
}

css

.foo .....   {   (NOT .weather)
...
}

you can ALSO use the ID value assigned to the module
the selector for id is

#id ... {
...
}

I use this cheatsheet to help me remember
https://www.w3schools.com/cssref/css_selectors.php

everthing before the {
is called the selector clause… because is ‘selects’ the elements of the page doc, that the stuff INSIDE the {} will be applied to

. = class
# = id
: is the prefix for special elements 
[ x ] means element with an attribute of x   <div x="foo"/>
nothing = tag name (table, ul, li, p, ....div)