doc: css: accessibility improvements

Minor tweaks to increase contrast as per WCAG AA guidelines.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

doc/_extensions/zephyr/domain/static/css/board-catalog.css

@@ -176,10 +176,8 @@
 
 .board-card .vendor {
   font-size: 12px;
-  color: var(--admonition-note-color);
-  font-weight: 900;
+  color: var(--body-color);
   margin-bottom: 18px;
-  opacity: 0.5;
 }
 
 .board-card img {

doc/_static/css/light.css

@@ -26,7 +26,7 @@
     --navbar-scrollbar-active-color: #7929d2;
     --navbar-scrollbar-background: #131e2b;
 
-    --link-color: #2980b9;
+    --link-color: #237ab3;
     --link-color-hover: #3091d1;
     --link-color-active: #105078;
     --link-color-visited: #9b59b6;
@@ -90,7 +90,7 @@
 
     --btn-neutral-background-color: #f3f6f6;
     --btn-neutral-hover-background-color: #e5ebeb;
-    --footer-color: #808080;
+    --footer-color: #747474;
 
     --graphviz-filter: none;
 }