Fix button contrast issues: add default colors to base .btn class and fix .cta-section .btn text visibility

- Base .btn now defaults to green bg (#148069) with white text
- .cta-section .btn on /tools page now explicitly sets color: white
- Fixes WCAG AA contrast failure where dark grey text (#444) was on dark blue bg (#144784)
- All bare .btn elements across site (tools, blog articles, error pages) now have visible text

assets/css/main.css

@@ -42,6 +42,8 @@ body {
     display: inline-flex;
     align-items: center;
     justify-content: center;
+    background: #148069;
+    color: white;
 }
 
 .btn-primary {