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STEP 1
Go to Template------Edit HTML-------Expand
Widget Templates
STEP 2
Search for ]]></b:skin>
by pressing Ctrl+F
STEP 3
STEP 3
Now paste the following codes just above ]]></b:skin>
.post{border:1px solid #E5E5E2;border-style:solid;border-radius:10px;
-moz-border-radius:10px;
-webkit-border-radius:10px;background:#ffffff;margin:.1em 0 .5em;padding:0px 8px 0px 8px}
-moz-border-radius:10px;
-webkit-border-radius:10px;background:#ffffff;margin:.1em 0 .5em;padding:0px 8px 0px 8px}
STEP 4
Now click on Preview and then Save Template
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1) To change the border color replace #E5E5E2 by your color code.
2) To adjust the border corner curves change 10px to your desired value.
3) To change the post background color change #ffffff by your color code.