Theming: Setting variables to null

Hi,

is there a simple way to nullify a variable?
When i set a variable to null before the @import ext4/default/all it gets reseted to the default value, since sass is treating null as an unassigned variable.

Code:

---

@import 'compass'

// Custom Variables
$base-color: #aa0000;
$grid-row-cell-selected-border-style: null;

@import 'ext4/default/all'

---

A structure like the following would be nice to have.
A switch to omit the import of the default variables whould work aswell.

Code:

---

@import 'compass'

//Custom Variables
$base-color: #aa0000;

//Defaults
@import 'ext4/default/variables'

//Nullify Variables
$grid-row-cell-selected-border-style: null;

@import 'ext4/default/all-without-variables

---

At the momemnt i copied the structure of ext4/default/all and made changes to it so its fits my needs.
But it would be nicer if there is a simpler way which doesn't involve changing existing files (Minimizing the effort to upgrade to newer ext versions).

Setting $grid-row-cell-selected-border-style to null isn't valid CSS then