Improve accessibility

Used Chrome Lighthouse to evaluate.

Author: mstathers

_layouts/default.html

@@ -47,13 +47,13 @@ <h1><a class="header" href=/>Stathers.net</a></h1>
     <div>
         <a class="img_link" style="float:left" href="https://github.com/mstathers/" target="_blank">
             <picture class="social_icon" alt="https://github.com/mstathers">
-                <source srcset="/css/GitHub-Mark-White-32px.png" media="(prefers-color-scheme: dark)" />
-                <img src="/css/GitHub-Mark-32px.png" />
+                <source srcset="/css/GitHub-Mark-White-32px.png" media="(prefers-color-scheme: dark)" alt="github log" width="32" height="32" />
+                <img src="/css/GitHub-Mark-32px.png" alt="github logo" width="32" height="32" />
             </picture>
         </a>
 
         <a class="img_link" style="float:left" href="https://www.linkedin.com/in/mstathers" target="_blank">
-            <img class="social_icon" src="/css/InBug-30px-R.png" alt="https://www.linkedin.com/in/mstathers">
+            <img class="social_icon" src="/css/InBug-30px-R.png" alt="https://www.linkedin.com/in/mstathers" width="39" height="30" />
         </a>
     </div>
 </div>

css/style.css

@@ -85,7 +85,7 @@ div.nav_ul_div {
 }
 
 div#page_date {
-    color: #7F7F7F;
+    color: #595959;
 }
 
 div#post {