Hi (sorry if this translation [Google translate] of my text in French is not optimal...), the upcoming macOS 26 Tahoe version allows users to completely hide the menu bar background for all applications. I find this effect quite “sexy” and attractive.
On macOS Sequoia (and certainly previous versions as well), it's possible to achieve "almost" the same "effect" with the Bartender app and a few adjustments (not that complicated to implement). Graphics software is still required, however.
Procedure:
1 – Choose a wallpaper with, if possible, a "fairly neutral, uniform, and continuous color" in its upper part (see illustration below).
2 – Bring the Finder to the foreground; this will make everything easier to do later.
3 – Open Bartender's Settings, click on "Menu Bar Style" in the Sidebar and choose the "Rounded separate sides" option. This will remove the background from the menu bar that doesn't contain any items. Still in Bartender, in the "Menu Bar Style" tab, choose the color white in "Menu bar tint" (do not choose a gradient).
4 – Now take a screenshot of this particular area, including your wallpaper and the end of the menu bar managed by Bartender.
6 – In your graphics software, open your wallpaper. Paste your previous screenshot and position it in its place, at the top.
7 – Create a vector rectangle spanning the entire width of your image and about a hundred pixels high (for a 5120 x 2880 px screen). Apply a vertical gradient of two identical colors using the color of the eyedropper (see illustration below). The idea is for the hue of this rectangle in its upper part to be very close to that of the screenshot. To blend the lower part of this gradient into the image, specify an opacity of 0.
8 – Save a copy of your wallpaper including the gradient rectangle you just created. In macOS system settings, choose this new wallpaper.
9 – Finally, in Bartender, return to the “Menu Bar Style” tab and slightly modify the “Menu bar tint” color to match the new wallpaper. Depending on the average color of the upper part of your wallpaper, you should aim for a slightly whiter color (in this case, a bluer color).
Note that the "Menu bar tint" color also, unfortunately, affects the brightness of the menu bar text (?!). The idea is therefore to ultimately choose a color that's as bright as possible, as close to white as possible.

On macOS Sequoia (and certainly previous versions as well), it's possible to achieve "almost" the same "effect" with the Bartender app and a few adjustments (not that complicated to implement). Graphics software is still required, however.
Procedure:
1 – Choose a wallpaper with, if possible, a "fairly neutral, uniform, and continuous color" in its upper part (see illustration below).
2 – Bring the Finder to the foreground; this will make everything easier to do later.
3 – Open Bartender's Settings, click on "Menu Bar Style" in the Sidebar and choose the "Rounded separate sides" option. This will remove the background from the menu bar that doesn't contain any items. Still in Bartender, in the "Menu Bar Style" tab, choose the color white in "Menu bar tint" (do not choose a gradient).
4 – Now take a screenshot of this particular area, including your wallpaper and the end of the menu bar managed by Bartender.
6 – In your graphics software, open your wallpaper. Paste your previous screenshot and position it in its place, at the top.
7 – Create a vector rectangle spanning the entire width of your image and about a hundred pixels high (for a 5120 x 2880 px screen). Apply a vertical gradient of two identical colors using the color of the eyedropper (see illustration below). The idea is for the hue of this rectangle in its upper part to be very close to that of the screenshot. To blend the lower part of this gradient into the image, specify an opacity of 0.
8 – Save a copy of your wallpaper including the gradient rectangle you just created. In macOS system settings, choose this new wallpaper.
9 – Finally, in Bartender, return to the “Menu Bar Style” tab and slightly modify the “Menu bar tint” color to match the new wallpaper. Depending on the average color of the upper part of your wallpaper, you should aim for a slightly whiter color (in this case, a bluer color).
Note that the "Menu bar tint" color also, unfortunately, affects the brightness of the menu bar text (?!). The idea is therefore to ultimately choose a color that's as bright as possible, as close to white as possible.