Formatting – Highlight the cell border in Google Docs

I'm trying to create a spreadsheet in Google Docs. I need a header at the top and a similar column at the left. To distinguish them from the rest of the table, I try them encourage the row that separates them from the rest of the table, such as the middle row below:

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When I make a selection of cells, a small triangle appears in the upper right cell of the selection. If I click on it, I can choose which margins are displayed, but I can not control their formatting. When I select the menus: Format -> Paragraph Styles -> Borders and Shading, I can make the margin "under" any cell bold, but this does not affect the border, but an area above it:
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How can I highlight only certain segments of table borders?