Fix btn-color

packages/preset-themes/src/styles/future.scss

@@ -1,3 +1,5 @@
+$min-contrast-ratio: 2;
+
 :root[data-bs-theme='dark'] {
   @import '@growi/core/scss/bootstrap/init-stage-1';
   @import '@growi/core/scss/bootstrap/theming/variables';