Tune box-shadow appearance in dark mode needs tuning

[chalin]

We should maybe use these as forwarded variables, at least so we can see their full values:

// scss-docs-start box-shadow-variables                                                                                                                                                                          
$box-shadow:                  0 .5rem 1rem rgba($black, .15) !default;
$box-shadow-sm:               0 .125rem .25rem rgba($black, .075) !default;
$box-shadow-lg:               0 1rem 3rem rgba($black, .175) !default;
$box-shadow-inset:            inset 0 1px 2px rgba($black, .075) !default;
// scss-docs-end box-shadow-variables    

The color needs adjustment. See assets/scss/_dark-adjustments.scss in the OTel repo for an example. Consider adding that to our dark-mode adjustments SCSS file.