Cannot create custom style without borders

I’m trying to create an EMPTY custom cell style (white cell without borders) but the changes I make to the borders of this style do not seem to save when I press OK.

I tried setting line thickness to 0,00pt, setting the line color as white and selecting the “no borders” border option, but it always reverts back to the By Default border (black 0,05pt line, which I cannot change either).

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,

Which LO and OS Version?

I asked a new question with more specifics and LO and OS versions as this one is quite frankly not clear and was created in a headache.
I also do not think it has to do with templates as this problem occurs in a blank document too. Maybe my Calc is just outdated ?