Improve export preview mode CSS variable value filling (#15110)

* 改进 CSS 变量替换为实际值

* Chrome、Edge、SiYuan 桌面端不需要替换 CSS 变量

* 外观模式或主题改变之后，重新加载所有将 CSS 变量替换为实际值的导出预览

* 代码块之间没有间距

* 去掉一个空格

* 使用 ua.Mobile() 判断移动设备

* 重构

kernel/api/export.go

@@ -29,6 +29,7 @@ import (
 	"github.com/88250/gulu"
 	"github.com/88250/lute/parse"
 	"github.com/gin-gonic/gin"
+	"github.com/mssola/useragent"
 	"github.com/siyuan-note/logging"
 	"github.com/siyuan-note/siyuan/kernel/model"
 	"github.com/siyuan-note/siyuan/kernel/util"
@@ -627,9 +628,22 @@ func exportPreview(c *gin.Context) {
 	}
 
 	id := arg["id"].(string)
-	stdHTML := model.Preview(id)
+
+	userAgentStr := c.GetHeader("User-Agent")
+	fillCSSVar := true
+	if userAgentStr != "" {
+		ua := useragent.New(userAgentStr)
+		name, _ := ua.Browser()
+		// Chrome、Edge、SiYuan 桌面端不需要替换 CSS 变量
+		if !ua.Mobile() && (name == "Chrome" || name == "Edge" || strings.Contains(userAgentStr, "Electron") || strings.Contains(userAgentStr, "SiYuan/")) {
+			fillCSSVar = false
+		}
+	}
+
+	stdHTML := model.Preview(id, fillCSSVar)
 	ret.Data = map[string]interface{}{
-		"html": stdHTML,
+		"html":       stdHTML,
+		"fillCSSVar": fillCSSVar,
 	}
 }