fix(Button): fix style for ghost + default

Author: anlyyao

src/button/button.less

@@ -92,7 +92,7 @@
 // light + text
 @button-light-text-color: var(--td-button-light-text-color, @brand-color);
 // ghost 优先级高于 theme，固定透明背景，白色边框，白色字体
-@button-ghost-color: var(--td-button-ghost-color, @bg-color-container);
+@button-ghost-color: var(--td-button-ghost-color, #fff);
 @button-ghost-border-color: var(--td-button-ghost-border-color, @button-ghost-color);
 @button-ghost-disabled-color: var(--td-button-ghost-disabled-color, @font-white-4);
 @button-ghost-hover-color: var(--td-button-ghost-hover-color, @font-white-2);