You can listen to the tune, titled Let's Keep It Going, below:
The Lopezes shared that they wanted to channel the chaos of the 90s to match how Wanda's world was unravelling. They also found this theme the hardest to wrote, having watched few of the 90s sitcoms since they were away at college.
In an interview with Marvel, the Lopezes discussed how the sitcom themes, which have been lauded as extremely catchy, had the common element of a tritone or 'devil's interval' - notably, the 'WandaVision' motif which was supposed to create a slight sense of unease.
They also noted how unlike their previous arrangements, where they wrote songs for others to sing, they mainly used their own vocals for the WandaVision themes (partly due to logistical constraints from the pandemic).
You can see a compilation of all the sitcom openings (including later episodes that feature spoilers) here. The tunes are made available on Marvel's VEVO channel a week after each episode is first uploaded.
[Latest episode opening factoid]And yes, that was Kathryn Hahn's voice singing during Agatha's intro.
Source: 1, 2, 3, 4, 5