fix(a11y): color contrast ratio fix (#1136)

Co-authored-by: llalunio <[email protected]>

Author: lyannel

source/assets/sass/components/_docsearch.scss

@@ -1,6 +1,12 @@
+@use '../config/color/brand';
+
 #docsearch {
   display: flex;
   flex-direction: column;
   justify-content: center;
   height: 100%;
 }
+
+.DocSearch-Button-Placeholder {
+  color: brand.$sl-color--midnight-blue;
+}