Fix: string index error causes failed load in kitty

```lua local red_green_string = hex:sub(1, 5) local blue_value = tonumber(hex:sub(6, 7), 16) ``` would read the first 5 digits as red-green and the last digit as blue then ```lua ans[accent] = string.format("%s%.2x", red_green_string, blue_value) ``` would format the 6th digit to be 2 characters wide. So the hex_to_rgb would parse a 7-digit hex code and crash