@hachesilva 
You can use 8digit hex color for lineHightlight, the last two digit is alpha level.

Here are two examples:
https://github.com/filmgirl/TextMate-Themes/blob/master/Django%20(Smoothy).tmTheme#L22
https://github.com/ajaxorg/ace/blob/master/tool/tmthemes/Textmate%20(Mac%20Classic).tmTheme