Are you using an old version? For recent versions the default thickness of border lines should be 0.75 pt.
However: The default for cell borders is ‘None.’ independent of the thickness.
In what way and on what occasions “reverts it back”? Please add precise information.
If you create a named custom style, this will not change the previous style of any cell. Only cells to which you appliy your custom style are afflicted.
New spreadsheet documents are created from a template. You may choose this template explicitly. Otherwise the Standard template is used if you defined one. If not an internal (“hard coded”) template is used.
Create a spreadsheet document containing the styles you want defined, in specific with the Default cell style you want, save it as a template, and make this template your standard,