The trick is to pass lv.PARTS.ITEMS and lv.STATE.CHECKED to tabview.set_style_....() function.
You can call any .set_style_...() function of the tabview object with those 2 “selectors”, and style will be applied to them.
Example (MicroPython): tab_btns.set_style_bg_color(lv.color_white(), lv.PART.ITEMS | lv.STATE.CHECKED)
In that you are correct, that the LV_PART_ITEMS selector is missing from the lv_tabviewdocumentation, but the tab buttons is a lv_btnmatrix, which has this selector, so we indirectly refer to buttons as “items”.
The example (from LVGL docs) contains the lv.PART.ITEMS and lv.STATE.CHECKED selectors also: