Style is not binding in css

Style is not binding in css

i am trying to bind style in my css style using below format but its not working and i am learner for web development can some one help me please what is mistack?

i am trying to bind style in my css style using below format but its not working and i am learner for web development can some one help me please what is mistack?

css

.block-header.row.sample h1{
        color: aqua;
    }

html

 <div class="block-header">
        <div class="row sample">
          <div class="col-sm-6">
            <h1 class="page-title">Pending Approvals</h1>
          </div>
       </div>
 </div>


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