perf: defined default app button style

app.wxss

@@ -17,4 +17,13 @@ page {
   display: flex;
   justify-content: flex-start;
   align-items: center;
+}
+.itt-btn {
+  background: var(--maincolor);
+  display: flex;
+  justify-content: center;
+  align-items: center;
+  color: #fff;
+  font-size: 26rpx;
+  font-weight: 400;
 }