CSS color variables, less bold font weight and more (#13567)

* CSS color variables, less bold font weight

- Define color variables for fully saturated colors and apply them where
  it made sense
- Add background color helper classes
- Globally reduce bold font weight from 700 to 500
- Remove border from timeline icons
- Unify dropzone styling
- Various border style consolidations

* attempt to fix test

* another attempt at tests

* fix contains

web_src/less/_markdown.less

@@ -26,7 +26,7 @@
   }
 
   .absent {
-    color: #cc0000;
+    color: var(--color-red);
   }
 
   .anchor {
@@ -77,7 +77,7 @@
   h6 {
     margin-top: 1em;
     margin-bottom: 16px;
-    font-weight: bold;
+    font-weight: 500;
     line-height: 1.4;
 
     &:first-of-type {
@@ -193,7 +193,7 @@
     margin-top: 16px;
     font-size: 1em;
     font-style: italic;
-    font-weight: bold;
+    font-weight: 500;
   }
 
   dl dd {
@@ -224,7 +224,7 @@
   }
 
   table th {
-    font-weight: bold;
+    font-weight: 500;
   }
 
   table th,
@@ -450,7 +450,7 @@
   }
 
   .csv-data th {
-    font-weight: bold;
+    font-weight: 500;
     background: #f8f8f8;
     border-top: 0;
   }