Borders enclose the paragraph area, i.e. they are drawn around the space defined by indents. You can’t specify a border larger or smaller than indents (you can add spacing, but it is used only to give more separation between paragraph contents and its border).
Indents are controlled by Indents & Spacing
of paragraph style dialog. Before text is for left indent, in LTR text, (or paragraph left “additional margin”) and After Text for right indent.
These can be set by ruler triangles (outside the style dialog), but using them is direct formatting and won’t be entered in the style definition.
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EDIT 2018-12-13
If the paragraph you want to apply the borders to is a Heading n paragraph, you face unwanted interactions between paragraph style settings ans list formatting. Remember that chapter numbering internally uses the list engine, whether you activate chapter numbers or not. Settings in Tools
>Chapter Numbering
take precedence over paragraph indents.
The left indent can be managed with Tools
>Chapter Numbering
, Position
tab. I’d recommend setting Aligned at and Indent at to the same value.
Unfortunately, there is no solution for right indent as resulted form my question here.