chore: 修改 backtop 文档

src/component/avatar/index.less

@@ -1,3 +1,5 @@
+@import "../../theme/variable.less";
+
 .layui-avatar {
   box-sizing: border-box;
   margin: 0;
@@ -19,7 +21,7 @@
   width: 32px;
   height: 32px;
   line-height: 32px;
-  border-radius: 4px;
+  border-radius: 2px;
 }
 
 .layui-avatar.layui-avatar-radius {

src/component/button/index.less

@@ -9,8 +9,8 @@
   text-align: center;
   white-space: nowrap;
   border: 1px solid transparent;
-  background-color: @button-primary-color;
-  border-radius: @button-border-radius;
+  background-color: @global-primary-color;
+  border-radius: 2px;
   cursor: pointer;
 }
 
@@ -50,15 +50,15 @@
 }
 
 .layui-btn-normal {
-  background-color: @global-normal-color;
+  background-color: #1e9fff;
 }
 
 .layui-btn-warm {
-  background-color: @global-warm-color;
+  background-color: #ffb800;
 }
 
 .layui-btn-danger {
-  background-color: @global-danger-color;
+  background-color: #ff5722;
 }
 
 .layui-btn-lg {

src/component/card/index.less

@@ -2,7 +2,7 @@
 
 .layui-card {
   margin-bottom: 15px;
-  border-radius: @card-border-radius;
+  border-radius: 2px;
   background-color: #fff;
   box-shadow: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
   .layui-card-header {