Use correct default color for new border config

[cderv]

fix #11561 and fix #11816

As discussed, the problem here seems to the wrong color for mixing.

When $body-color and $body-bg are not defined, we should use the same values as Bootstrap default variables, which are the ones used for default theme (i.e theme: default)

However, we need to try get back a nice default. Previously $border-color used was bootstrap default

quarto-cli/src/resources/formats/html/bootstrap/dist/scss/_variables.scss

Line 550 in 768e540

$border-color: $gray-300 !default;

quarto-cli/src/resources/formats/html/bootstrap/dist/scss/_variables.scss

Line 12 in 768e540

$gray-300: #dee2e6 !default;

One way to fix is the following:

• We still follow the mix() idea for $border-color introduced in
  • blend colors against background instead of hard-coding to lighten #11283
• But we use the default values if body-color or body-bg is not defined.

So

quarto-cli/src/resources/formats/html/bootstrap/_bootstrap-variables.scss

Lines 277 to 281 in 768e540

	$border-color: mix(
	if(variable-exists(body-color), $body-color, $gray-900),
	if(variable-exists(body-bg), $body-bg, $white),
	10%
	) !default;

This way we get

Which is quite close.

If we want to keep default theme the same, we could just mix() when variable exists.

@if variable-exists(body-color) and variable-exists(body-bg) {
  $border-color: mix($body-color, $body-bg, 10%) !default;
  $table-border-color: $border-color !default;
}

But I don't think we want to make use of those @if because our SCSS -> CSS exposer logic.

This PR aims to fix the default theme. Then we need to fix

• Tabsets borders are not displayed correctly on some non-default theme #11816
  before we can do thorough testing with various theme.

Right now this is some examples after this PR

• Vapor
  
• Darkly
  
• Cyborg
  
• litera
  
• lux
  

So I think we get good mix with this.

[cscheid]

I think we should backport this one to 1.6.

[cderv]

Yes agreed. It seems you are ok with the color mix. I'll backport tomorrow, and think of something regarding testing. I think I can test with playwright at least the default theme.