[a11y] Increase contrast on some colors for WCAG conformance (#1996)

Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

src/pretix/static/bootstrap/scss/bootstrap/_variables.scss

@@ -208,7 +208,7 @@ $input-border-radius-small:      $border-radius-small !default;
 $input-border-focus:             #66afe9 !default;
 
 //** Placeholder text color
-$input-color-placeholder:        #999 !default;
+$input-color-placeholder:        #767676 !default;
 
 //** Default `.form-control` height
 $input-height-base:              ($line-height-computed + ($padding-base-vertical * 2) + 2) !default;

src/pretix/static/pretixbase/scss/_variables.scss

@@ -9,19 +9,19 @@
 $gray-darker: lighten(#000, 13.5%);
 $gray-dark: lighten(#000, 20%);
 $gray: lighten(#000, 33.5%);
-$gray-light: lighten(#000, 60%);
+$gray-light: lighten(#000, 55%);
 $gray-lighter: lighten(#000, 93.5%);
 $gray-lightest: lighten(#000, 97.25%);
 
 $font-family-sans-serif:  "Open Sans", "OpenSans", "Helvetica Neue", Helvetica, Arial, sans-serif !default;
 $text-color: #222222 !default;
-$text-muted: #999999 !default;
+$text-muted: #767676 !default;
 
 $brand-primary: #7f5a91 !default;
 $brand-success: #50a167 !default;
 $brand-info: #5f9cd4 !default;
 $brand-warning: #ffb419 !default;
-$brand-danger: #d36060 !default;
+$brand-danger: #c44f4f !default;
 
 $btn-default-border: #CCCCCC;
 

src/pretix/static/pretixcontrol/scss/_forms.scss

@@ -1,3 +1,7 @@
+.help-block {
+  color: $text-muted;
+}
+
 td > .form-group {
   margin-bottom: 0;
 }

src/pretix/static/pretixpresale/scss/_forms.scss

@@ -1,3 +1,7 @@
+.help-block {
+  color: $text-muted;
+}
+
 a.btn, button.btn {
   max-width: 100%;
   white-space: normal;