Remove hard-coded colors which break dark mode

The algorithm and internal slot boxes had hard-coded CSS background
colors set which didn't adjust when the page was viewed in dark mode.

They are simply removed as they don't seem necessary.

index.bs

@@ -116,7 +116,6 @@ summary {
 
 div.algorithm-steps {
     padding: .5em;
-    background-color: ghostwhite;
 }
 
 .algorithm-steps {
@@ -142,7 +141,6 @@ div.informalsteps {
     padding: .5em;
     border: thin solid #88e !important;
     border-radius: .5em;
-    background-color: ghostwhite;
 }
 
 .informalsteps {
@@ -167,7 +165,6 @@ div.internal-slots {
     padding: .5em;
     border: thin solid #88e !important;
     border-radius: .5em;
-    background-color: aliceblue;
 }
 
 .internal-slots {