Issue #3040241 by bnjmnm, lauriii, DyanneNova: Add section and add block...

Issue #3040241 by bnjmnm, lauriii, DyanneNova: Add section and add block section doesn't have sufficient color contrast for text

core/misc/icons/686868/plus.svg

+<svg xmlns="http://www.w3.org/2000/svg" width="16px" height="16px"><path fill="#686868" d="M0.656,9.023c0,0.274,0.224,0.5,0.499,0.5l4.853,0.001c0.274-0.001,0.501,0.226,0.5,0.5l0.001,4.853 c-0.001,0.273,0.227,0.5,0.501,0.5l1.995-0.009c0.273-0.003,0.497-0.229,0.5-0.503l0.002-4.806c0-0.272,0.228-0.5,0.499-0.502 l4.831-0.021c0.271-0.005,0.497-0.23,0.501-0.502l0.008-1.998c0-0.276-0.225-0.5-0.499-0.5l-4.852,0c-0.275,0-0.502-0.228-0.501-0.5 L9.493,1.184c0-0.275-0.225-0.499-0.5-0.499L6.997,0.693C6.722,0.694,6.496,0.92,6.495,1.195L6.476,6.026 c-0.001,0.274-0.227,0.5-0.501,0.5L1.167,6.525C0.892,6.526,0.665,6.752,0.665,7.026L0.656,9.023z"/></svg>

core/modules/layout_builder/css/layout-builder.css

@@ -14,10 +14,10 @@
 }
 
 .layout-builder__link--add {
-  color: #787878;
+  color: #686868;
   border-bottom: none;
   padding-left: 24px;
-  background: url(../../../misc/icons/787878/plus.svg) transparent top left / 16px 16px no-repeat;
+  background: url(../../../misc/icons/686868/plus.svg) transparent top left / 16px 16px no-repeat;
 }
 
 .layout-builder__link--add:hover,