fix(preset-shiny): Fix border of radio and checkbox buttons

[gadenbuie]

Before

Note light border on checkbox and radio buttons.

After

Notes

The border of checkbox and radio buttons is driven by $form-check-input-border which uses var(--bs-border-color) by default (the same as the card border).

bslib/inst/lib/bs5/scss/_variables.scss

Line 958 in 2e6b202

$form-check-input-border: var(--#{$prefix}border-width) solid var(--#{$prefix}border-color) !default;

Input borders (select, text input) are set via $input-border-color

bslib/inst/lib/bs5/scss/_variables.scss

Line 914 in 2e6b202

$input-border-color: var(--#{$prefix}border-color) !default;

which we override in the Shiny preset.

bslib/inst/builtin/bs5/shiny/_variables.scss

Line 89 in 2e6b202

$input-border-color: $gray-500 !default;