As a Roblox developer, it is currently impossible to disable and or change the colors of the text box selection.
If Roblox is able to address this issue, it would improve my game / my development experience because not only could I change the color of selection boxes and the text selection color to match UI themes but being able to turn them off would prevent the text being visible when selecting even if the text transparency is set to 1.
A few days ago I was creating a password text box with a text label and a text box with the text transparency set to 1. The text label was for masking the text entered while the text box would be what I took the password text from. While selecting the text from the text box, the text appeared and it was right under the text label so it looked very weird. Being able to further modify the selection boxes would not only solve this problem but would also make the selection fit in more with the UI if developers could change the selection color and the selection text color.