style: set default color for the font highlighting

Author: peterpeterparker

webcomponents/highlight-code/src/components/highlight-code/deckdeckgo-highlight-code.scss

@@ -57,7 +57,7 @@ div.deckgo-highlight-code-container {
     }
 
     span.deckgo-highlight-code-line {
-      background: var(--deckgo-highlight-code-line-background);
+      background: var(--deckgo-highlight-code-line-background, #3880ff);
       padding: var(--deckgo-highlight-code-line-padding);
       border-top: var(--deckgo-highlight-code-line-border-top);
       border-bottom: var(--deckgo-highlight-code-line-border-bottom);

webcomponents/highlight-code/src/index.html

@@ -12,7 +12,7 @@
 
 <h1>Editable:</h1>
 
-<deckgo-highlight-code language="java" editable="true" highlight-lines="0,2 6,6" style="--deckgo-highlight-code-line-background: lightgrey;">
+<deckgo-highlight-code language="java" editable="true" highlight-lines="0,2 6,6">
   <code slot="code">interface NumericTest {
     boolean computeTest(int n);
     }