[A11y] Improve color contrast of grey buttons (#8259)

Addresses #8192

You can find the test plan here: #8192 (comment)

Author: loic-sharma

src/Bootstrap/dist/css/bootstrap.css

@@ -150,8 +150,8 @@
 @btn-font-weight:                normal;
 
 @btn-default-color:              #333;
-@btn-default-bg:                 #f4f4f4;
-@btn-default-border:             #f4f4f4;
+@btn-default-bg:                 #fff;
+@btn-default-border:             #7f7f7f;
 
 @btn-primary-color:              #fff;
 @btn-primary-bg:                 @brand-primary;

src/Bootstrap/less/variables.less

@@ -44,7 +44,7 @@
                             </select>
                         </div>
                         <div class="text-right col-xs-1">
-                            <button class="btn" type="submit" title="Add new member" aria-label="Add new member" data-bind="click: AddMember">Add</button>
+                            <button class="btn btn-primary" type="submit" title="Add new member" aria-label="Add new member" data-bind="click: AddMember">Add</button>
                         </div>
                     </div>
                 </div>