Less than you think. Having some general ideal about the main asset classes and how finance works is useful but don't feel like you need a CFA or anything. Same goes for game theory, helpful especially on the protocol design side of things but don't feel like you need to be an expert to get started.

More useful coming from webdev would be good solidity [1] skills. There is a real shortage currently. Most general engineering practice carries over. Threat modelling is useful and more generally having a good understand about security. You should know about unit tests, code coverage and how they fit into building robust code.

In term of tools and frameworks. Hardhat [2] in my experience is a little nicer to use than truffle [3] but both help loads. Knowing how to use the Openzeppelin [4] contracts would be very handy. Waffle [5] for testing is helpful. Understanding about ERC-20 [6], ERC-721 [7] and ERC-1155 [8].

Cryptozombies [9] is a decent tutorial to get you started. Rust is also useful to know for Polkadot [10] (for ink! [11]) and other chains. The DeFi developer [12] roadmap seems decent for more resources.

[1] https://soliditylang.org/

[2] https://hardhat.org/

[3] https://www.trufflesuite.com/

[4] https://openzeppelin.com/contracts/

[5] https://getwaffle.io/

[6] https://eips.ethereum.org/EIPS/eip-20

[7] https://eips.ethereum.org/EIPS/eip-721

[8] https://eips.ethereum.org/EIPS/eip-1155

[9] https://cryptozombies.io/

[10] https://polkadot.network/

[11] https://github.com/paritytech/ink

[12] https://github.com/OffcierCia/DeFi-Developer-Road-Map