Hello @Nuno Gonçalves,
Regarding the colors of the column headers, could you please confirm if this is a possible solution?
If you want to change the background-color of the column headers:
.wj-flexgrid .wj-colheaders > .wj-row > .wj-cell.wj-header {
background-color: blue;
}
If you want to change the background-color of the row headers:
.wj-flexgrid .wj-rowheaders > .wj-row > .wj-cell.wj-header {
background-color:red;
}
If you want to change the background-color of top-left cells:
.wj-flexgrid .wj-topleft > .wj-row > .wj-cell.wj-header {
background-color: green;
}
This way you will end up with the following configuration:

You can also change all of them by using:
.wj-flexgrid .wj-colheaders > .wj-row > .wj-cell.wj-header,
.wj-flexgrid .wj-rowheaders > .wj-row > .wj-cell.wj-header,
.wj-flexgrid .wj-topleft > .wj-row > .wj-cell.wj-header {
background-color: blue;
}
Or:
.wj-flexgrid .wj-row > .wj-cell.wj-header {
background-color:blue;
}
This way you will end up with the following configuration:

Hope this answer is useful.
Thank you,
Tiago Pereira