@Amaury
Are you working from a already customized style? The reason I ask, is if you want to farther down the road have a different type of layout than what your parent (framework) has then you will be working backwards. I would definitely suggest starting with an unedited style to start your framework.
Also something I found useful for things such as this example. You make a property to control an area that doesn't already have a property you have problems of not all of the CSS being applied as it gets overwritten with the original CSS. What we have done to remedy this is to add a new selector to the html so we can target that and be more specific.
It just makes it easier to remember and always something you can reference in your CSS instead of searching through for specifics.
Code:
.XenBase .AvatarEditor .avatarOption {
background-color: @primaryLightest;
border-color: @primaryLight;
}