refactor CSS to use cascade layers

[aqandrew]

Adam Argyle stated in a video that cascade layers are a helpful CSS feature for declaring the order in which styles will be overwritten.

These sequential imports can then be condensed into a single line of CSS:

aqandrew.com/src/layouts/BaseLayout.astro

Lines 5 to 7 in 6ae39f2

	import '../assets/css/variables.css';
	import '../assets/css/global.css';
	import '../assets/css/utils.css';

[aqandrew]

Resources for importing into cascade layers:

https://youtu.be/DI-dgSlaeIY?si=y1WZBXW3qf2XuqCK&t=94

https://developer.mozilla.org/en-US/docs/Web/CSS/@import#importing_css_rules_into_a_cascade_layer