[A11Y] Fix some Website Accessibility Issues (#276)

* [A11Y] Fix some Website Accessibility Issues
Fixes #275

* Update foregroud color `--pico-color-sand-100`

* Add alt attribute specifies an alternate text for an image

* Update assets/css/pico.colors.min.css

* [WCAG 2 AA] Select the color contrast for the <a> elements

Author: oleksis

_layouts/index.html

@@ -8,7 +8,7 @@
   </article>
   {{ content }}
 
-  <img src="{{'/assets/images/dcus.jpg' | relative_url }}" />
+  <img src="{{'/assets/images/dcus.jpg' | relative_url }}"  alt="DjangoCon US 2023"/>
 
   {%- include latest-posts.html -%}
   {%- include join_us.html -%}

assets/css/bpd.css

@@ -55,6 +55,13 @@ input::-moz-focus-inner {
   padding: 0;
 }
 
+/*
+a.contrast, a.secondary {
+  color: #00d7ff;
+  text-decoration: none;
+}
+ */
+
 div#newsletter {
   margin: 1rem 0;
   padding: 1rem;
@@ -90,8 +97,8 @@ div#newsletter {
 .back-to-top:active,
 .back-to-top:focus,
 .back-to-top:hover {
-  border-color: #2d72d9;
-  color: #2d72d9;
+  border-color: #3588ff;
+  color: #3588ff;
   outline: 0;
 }
 
@@ -107,7 +114,7 @@ div#newsletter {
 }
 
 .menu-item a {
-  color: #70777d;
+  color: #868e96;
   display: inline-block;
   font-size: 0.88889rem;
   font-weight: 700;
@@ -122,7 +129,7 @@ div#newsletter {
 .menu-item a:hover,
 .menu-item.nav-current a:link,
 .menu-item.nav-current a:visited {
-  color: #2d72d9;
+  color: #3588ff;
 }
 
 .icon-menu {
@@ -214,7 +221,7 @@ a.logo-text img {
 }
 
 .site-title a:hover {
-  color: #2d72d9;
+  color: #3588ff;
 }
 
 .site-navigation {

assets/css/pico.colors.min.css

@@ -340,7 +340,7 @@
   --pico-color-sand-250: #beb8a7;
   --pico-color-sand-200: #ccc6b4;
   --pico-color-sand-150: #dad4c2;
-  --pico-color-sand-100: #e8e2d2;
+  --pico-color-sand-100: #f3eddc;
   --pico-color-sand-50: #f2f0ec;
   --pico-color-sand: #ccc6b4;
   --pico-color-grey-950: #111111;
@@ -403,7 +403,7 @@
   --pico-color-slate-100: #dfe3eb;
   --pico-color-slate-50: #eff1f4;
   --pico-color-slate: #525f7a;
-  --pico-color-light: #fff;
+  --pico-color-light: #03d5ff;
   --pico-color-dark: #000;
 }
 .pico-color-red-950 {