DevControl rounded corners are white

[robb-j]

The structure is:

.devControl
  .devControl-panel
    .panel.is-warning
      .devControl-heading
      .devControl-block

The devControl-panel has the white background which means the border radius on devControl-heading and devControl-block shows white, not transparent.

The white background needs moving to the devControl-block class

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Also I think .panel.is-warning is not needed anymore