I want to make diy lights for my closet, when I push my slide door open the lights will turn on and when I close the door a pushbutton will be pressed and the lights will turn off, but I don't want to use a microcontroller, so can you recommend me a transistor that would turn on the lights when the pushbutton isn't pressed and turn them off when the pushbutton is pressed or if there is even an easier way, the led strip only consumes around 100 mA at 12V
Just place a switch of your choice in series with your LED string. Any of several push button or lever actuated micro switches will work. You want a N/C (Normally Closed) switch. The door holds the switch active and Open, when you open the closet door the switch closes. There is no need for a switching circuit using a micro-controller, transistor or anything else. Look for Micro Switches or Push Button switches, most come with a Common and N/O and N/C contact.
Just make sure whatever switch you choose is rated for your DC current and Voltage.
There are super simple slider door switches that include a magnet to activate a switch. The magnet should be installed in the moving part of the door - no moving pieces. Super easy.