Set font-sizes relative to font-size-base (#24060)

* font-sizes relative to font-size-base

* ditch $heading-font-size-base, simplify

* apply same to .lead var

* also update progress bar font-size var

scss/_variables.scss

@@ -212,9 +212,9 @@ $font-family-monospace: "SFMono-Regular", Menlo, Monaco, Consolas, "Libera
 $font-family-base: $font-family-sans-serif !default;
 // stylelint-enable value-keyword-case
 
-$font-size-base:  1rem !default; // Assumes the browser default, typically `16px`
-$font-size-lg:  1.25rem !default;
-$font-size-sm:  .875rem !default;
+$font-size-base: 1rem !default; // Assumes the browser default, typically `16px`
+$font-size-lg: ($font-size-base * 1.25) !default;
+$font-size-sm: ($font-size-base * .875) !default;
 
 $font-weight-light: 300 !default;
 $font-weight-normal: 400 !default;
@@ -223,12 +223,12 @@ $font-weight-bold: 700 !default;
 $font-weight-base: $font-weight-normal !default;
 $line-height-base: 1.5 !default;
 
-$h1-font-size:  2.5rem !default;
-$h2-font-size:  2rem !default;
-$h3-font-size:  1.75rem !default;
-$h4-font-size:  1.5rem !default;
-$h5-font-size:  1.25rem !default;
-$h6-font-size:  1rem !default;
+$h1-font-size: $font-size-base * 2.5 !default;
+$h2-font-size: $font-size-base * 2 !default;
+$h3-font-size: $font-size-base * 1.75 !default;
+$h4-font-size: $font-size-base * 1.5 !default;
+$h5-font-size: $font-size-base * 1.25 !default;
+$h6-font-size: $font-size-base !default;
 
 $headings-margin-bottom: ($spacer / 2) !default;
 $headings-font-family: inherit !default;
@@ -247,8 +247,8 @@ $display3-weight: 300 !default;
 $display4-weight: 300 !default;
 $display-line-height: $headings-line-height !default;
 
-$lead-font-size:  1.25rem !default;
-$lead-font-weight:  300 !default;
+$lead-font-size: ($font-size-base * 1.25) !default;
+$lead-font-weight: 300 !default;
 
 $small-font-size: 80% !default;
 
@@ -707,15 +707,15 @@ $alert-border-width: $border-width !default;
 
 // Progress bars
 
-$progress-height:  1rem !default;
-$progress-font-size:  .75rem !default;
-$progress-bg:  $gray-200 !default;
-$progress-border-radius:  $border-radius !default;
-$progress-box-shadow:  inset 0 .1rem .1rem rgba($black,.1) !default;
-$progress-bar-color:  $white !default;
-$progress-bar-bg:  theme-color("primary") !default;
-$progress-bar-animation-timing:  1s linear infinite !default;
-$progress-bar-transition:  width .6s ease !default;
+$progress-height: 1rem !default;
+$progress-font-size: ($font-size-base * .75) !default;
+$progress-bg: $gray-200 !default;
+$progress-border-radius: $border-radius !default;
+$progress-box-shadow: inset 0 .1rem .1rem rgba($black,.1) !default;
+$progress-bar-color: $white !default;
+$progress-bar-bg: theme-color("primary") !default;
+$progress-bar-animation-timing: 1s linear infinite !default;
+$progress-bar-transition: width .6s ease !default;
 
 // List group