Hi Abdelfattah,

you can use ".SFC.SourceCode-Forms-Controls-Web-DataLabel" class to change style of all datalabel available with in the forms,  but in case if you are targetting only specific datalabel then  use below selector

<style>
.SFC.SourceCode-Forms-Controls-Web-DataLabel{
 text-align: left;
}</style>

select datalabel in designer canvas whome you want to give style than on right side in properties for  under Detail for Name

 provide some text for example lblusername  and than  use below css to change its style

<style>
spanename="lblusername"]{
 text-align: left;
}</style>

Note red highlited text will be value assigned to name property.

If text-align: left;  css property dosen't work here, pls check  css style under properties and also for tabel cell (to check for table cell first select cell and than make sure align left is selected.) 

Let me know if this helps you.