[OnePlus 7 Pro Themes, Apps, and Mods] AhsanU:
It's not very difficult at all and is nothing to be fearful of. The beauty of OnePlus and this community is that it's difficult to royally screw up to a point of no return. If your device can get into fastboot, you're always fine. If you can get into EDL mode, you're fine. Here's a small guide of what you have to do, in order: I'm writing it out as if you are completely new in case you are or if anyone reading it is, I'm not assuming you are and don't mean to disrespect you if you aren't.

1) Download and install "Minimal ADB and Fastboot" on your computer from here: https://xdaforums.com/showthread.php?t=2317790

2) Go to your phone's settings -> about phone -> then tap the build number until you've unlocked developer options.

3) Go to developer options under settings -> system -> developer options. Enable OEM unlocking, then scroll down and enable USB debugging. Connect your phone to your computer and authorise access on your phone when it prompts for it.

4) On your computer, go to the folder where you installed Minimal ADB and Fastboot. Hold your shift key and right click anywhere where there's open space (without a selected file in the window), and hit "Open Powershell Window Here"

5) Confirm your device is indeed connected by typing "adb devices". On my computer, I'm forced to type "./adb devices" with the period and forward flash for whatever reason, but the command prompt should show your device as connected. Might be a Windows 10 thing, I haven't researched it.

6) In the command prompt you've just opened, type "adb reboot-bootloader". You're now in fastboot mode.

7) Once you are in fastboot, type "fastboot oem unlock". Follow the prompts on your phone to unlock the bootloader with the volume keys. This will erase all your data on your phone. Your phone will now reboot after you've confirmed that you're okay with data being wiped.

8) Set up your phone again and unlock developer options/USB debugging again.

9) On your computer, download the correct patched version of your boot.img here: https://xdaforums.com/oneplus-7-pro/how-to/guide-root-oneplus-7-pro-patched-boot-t3931205 - please remember to download the correct version, downloading the wrong one and flashing it will cause your WiFi not to work. You can check which version you need by going into settings -> about phone -> build number. I'm on Oxygen OS 9.5.3. If you've accidentally installed the wrong one, no worries - download the correct one and follow the steps again.

10) Drag the file you've just downloaded (filename should be: boot_patched.img) into your Minimal ADB and Fastboot folder.

11) Hold your shift key + right click anywhere in the window and open Powershell again.

12) Reboot into bootloader again by typing "adb reboot-bootloader". Again, if you're on Windows 10 or if you get some long message, you may have to type "./adb reboot-bootloader". This applies to all commands input into the command prompt, including the next steps'.

13) Once your phone has reboot into fastboot, in the command prompt, type "fastboot flash boot boot_patched.img".

14) After that's completed, type "fastboot reboot". Your phone will now reboot normally.

15) On your phone, download Magisk Manager from here: https://www.xda-developers.com/how-to-install-magisk/ - it's a 1.85 mb file that's actually just an apk file. Allow Google Chrome access to your storage, and allow Chrome to install apps from an unknown source - then continue to install Magisk Manager.

16) Once installed, you'll be prompted to finish installing Magisk, tap continue and let it do its thing. Then hit reboot.

17) Once rebooted, locate Magisk Manager and open the app. You should see the fact that it successfully installed. From there, swipe in from the left and select downloads to start looking for mods! You can download them on Chrome and find them under the modules, just gotta click the plus sign and locate it.

Cheers.