Add new CSS variables --color-accent and --color-small-accent (#21305)

At the moment, this is only used to replace the color of the `viewed`
checkbox and of the `has changed` label.
Previously, the used variable accentuated always either darker or
lighter, which meant that one theme looked good while the other didn't.

Co-authored-by: silverwind <me@silverwind.io>

web_src/less/_base.less

@@ -159,6 +159,9 @@
   --color-tooltip-text: #ffffff;
   --color-header-bar: #ffffff;
   --color-label-active-bg: #d0d0d0;
+  /* accent */
+  --color-small-accent: var(--color-primary-light-6);
+  --color-accent: var(--color-primary-light-4);
   /* backgrounds */
   --checkbox-mask-checked: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="-1 -1 18 18" width="16" height="16"><path fill-rule="evenodd" d="M13.78 4.22a.75.75 0 010 1.06l-7.25 7.25a.75.75 0 01-1.06 0L2.22 9.28a.75.75 0 011.06-1.06L6 10.94l6.72-6.72a.75.75 0 011.06 0z"></path></svg>');
   --checkbox-mask-indeterminate: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" width="16" height="16"><path fill-rule="evenodd" d="M2 7.75A.75.75 0 012.75 7h10a.75.75 0 010 1.5h-10A.75.75 0 012 7.75z"></path></svg>');

web_src/less/_review.less

@@ -264,11 +264,12 @@ a.blob-excerpt:hover {
 }
 
 .changed-since-last-review {
-  margin: 0 5px;
-  padding: 0 3px;
-  border: 2px var(--color-primary-light-3) solid;
-  background-color: var(--color-primary-alpha-30);
-  border-radius: 7px;
+  border: 1px var(--color-accent) solid;
+  background-color: var(--color-small-accent);
+  border-radius: 15px;
+  padding: 4px 8px;
+  margin: -8px 0; // just like other buttons in the diff box header
+  font-size: .857rem; // just like .ui.tiny.button
 }
 
 .viewed-file-form {
@@ -286,8 +287,8 @@ a.blob-excerpt:hover {
 }
 
 .viewed-file-checked-form {
-  background-color: var(--color-primary-light-6);
-  border: 1px solid var(--color-primary-light-4);
+  background-color: var(--color-small-accent);
+  border: 1px solid var(--color-accent);
 }
 
 #viewed-files-summary {

web_src/less/themes/theme-arc-green.less

@@ -134,6 +134,8 @@
   --color-reaction-active-bg: var(--color-primary-alpha-40);
   --color-header-bar: #2e323e;
   --color-label-active-bg: #4c525e;
+  --color-small-accent: var(--color-primary-light-5);
+  --color-accent: var(--color-primary-light-3);
 }
 
 ::-webkit-calendar-picker-indicator {