highlight fixes for a11y and arrow in nojs

these more specific and bad highlight styles bleed through from dark to light mode

fixes #12399
fixes #12522

Author: gordonwoodhull

src/resources/pandoc/highlight-styles/a11y-dark.theme

@@ -155,7 +155,6 @@
             "underline": false
         },
         "Import": {
-            "text-color": "#f8f8f2",
             "background-color": null,
             "bold": false,
             "italic": false,

src/resources/pandoc/highlight-styles/arrow-dark.theme

@@ -1,8 +1,6 @@
 {
     "text-styles": {
         "Alert": {
-            "background-color": "#2a0f15",
-            "bold": true,
             "selected-text-color": "#f07178",
             "text-color": "#f07178"
         },