A 500kb game should load in 20 seconds on a decent 3G signal. The average size of the top 1000 websites is apparently 1.2mb. Whoever is imposing a 250kb restriction is asking you to produce a working game in about one fifth the size of the average website, which is a very unusual limitation to put in place. Either way the compressed script is only 35kb so represents only 14% of your budget.
We are not designing a game engine for such extreme requirements. It should be obvious that a game is going to be a fair bit larger than a website. Perhaps you need to hand-code the game to meet such stringent limits.
FWIW C2 does not use CSS at all (all canvas-based), and I am doubtful your own PNG compression can beat what Construct 2 already does - be sure to double check you're actually reducing sizes and not using a less optimised tool that is producing larger PNGs.