I can probably find some good blog posts that explain everything on my PC, but the basic problem with them is the omega-6 to omega-3 ratio. They’re used to create pro-inflammatory and anti-inflammatory signalling molecules respectively (lookup eicosanoids on Wiki), and a ratio of around 2:1 (the exact number is debated) is needed for health. Most vegetable oils are at least 60% omega-6, with generally negligible amounts of omega-3. Given the lack of fatty fish and other sources of omega-3 in the diet, it’s no surprise Americans are estimated to have a ratio as high as 22:1.
Omega-3 fatty acids are also extremely sensitive to light, heat and air, so any contained in heavily processed vegetable oils (basically all of them) will be denatured and oxidised and more harmful than good. The oxidation and other “leftovers” from processing like traces of solvents and heavy metals are another reason they’re so unhealthy. Margarine still contains trans-fats and is literally closer to plastic than anything you can call food. Avoid them all like the plague. Virgin olive oil is unprocessed and contains less than 10% omega-6 content, so it’s fine to use.
As far as candida goes, I believe it gets on well with omega-6s and can use them to construct biofilms. I don’t know any more than that, but you might find something on Google or PubMed if you search “candida PUFAs” or something. I think you mentioned depression before, so you might be interested in Mental Health and Omega-3/6 Ratio that I posted in the stress thread. There’s a lot more to an omega-3 deficiency than just inflammation.