Controls and search in Rustdoc header are visually distracting

[lambda]

This is being broken out of #59829 to provide smaller, actionable items that can be independently discussed and worked on.

A visual inspection of the Rustdoc design reveals that there are a lot of distracting design elements, which can be especially problematic for people with dyslexia or attention disorders.

One of the first encountered is the header containing the search bar and controls. It is visually larger than the main heading of the page, it contains several high contrast icons and controls which also have additional bounding boxes, the whole thing presents a non-rectangular shape making it stand out more, and the search box contains a fairly verbose set of instructions.

I have a few suggestions to improve the situation; some seem like obvious improvements, some are more debatable and I'm open to other suggestions:

• Drop the horizontal rule, or make it span the full width and low contrast like the mdBook style after scrolling down Remove horizontal lines at top of page #92797
• Drop the boxes around the icons
• Make the icons and "All Crates" control lower contrast
• Increase the size of the main heading of the page to be larger than the search and controls heading
• Consider removing the "All Crates" dropdown; I am not sure it is pulling its weight. If it is useful, it could instead be an option that appears after doing a search, rather than appearing on every page. Move crate drop-down to search results page #92490
• Consider moving theme and settings into a single menu Pick themes on settings page, not every page #92629
• Consider simplifying text to simply say "Search" and providing help elsewhere, such as in the above single menu
• Consider making the header span the whole width of the page, rather than appearing to be nested within the content by being inside the area delimited by the sidebar. This would be a significant change to the overall Rust docs style, but seems more consistent with the way many other sites are laid out.

Why are the buttons for theme selection and doc settings part of the search box in the first place? I think they should be removed.

I'd suggest that the search box appears like this:

Until it is focused, in which case it should look like this:

The changes:

• Removed the horizontal rule and the two buttons
• Hidden the 'All crates' dropdown by default and placed it second to the search box
• Reduced the font size to be less prominent but still readable
• Used the same font and font size in the input box and the dropdown
• Given the box more contrast when it is focused