Add darkmode variant for black-normal color

This resolves the issue where font with color black-normal
was not readable in dark-mode.

Changelog: fixed

app/assets/stylesheets/startup/startup-dark.scss

@@ -1776,6 +1776,7 @@ body.gl-dark {
   --border-color: #4f4f4f;
   --white: #333;
   --black: #fff;
+  --black-normal: #fafafa;
   --svg-status-bg: #333;
 }
 .nav-sidebar li a {
@@ -2001,6 +2002,7 @@ body.gl-dark {
   --border-color: #4f4f4f;
   --white: #333;
   --black: #fff;
+  --black-normal: #fafafa;
   --svg-status-bg: #333;
 }
 .tab-width-8 {

app/assets/stylesheets/themes/_dark.scss

@@ -93,6 +93,7 @@ $gray-darker: #4f4f4f;
 $gray-darkest: #c4c4c4;
 
 $black: #fff;
+$black-normal: $gray-900;
 $white: #333;
 $white-light: #2b2b2b;
 $white-normal: #333;
@@ -192,6 +193,7 @@ body.gl-dark {
 
   --white: #{$white};
   --black: #{$black};
+  --black-normal: #{$black-normal};
 
   --svg-status-bg: #{$white};
 

ee/app/assets/stylesheets/startup/startup-dark.scss

@@ -1776,6 +1776,7 @@ body.gl-dark {
   --border-color: #4f4f4f;
   --white: #333;
   --black: #fff;
+  --black-normal: #fafafa;
   --svg-status-bg: #333;
 }
 .nav-sidebar li a {
@@ -2001,6 +2002,7 @@ body.gl-dark {
   --border-color: #4f4f4f;
   --white: #333;
   --black: #fff;
+  --black-normal: #fafafa;
   --svg-status-bg: #333;
 }
 .tab-width-8 {