Improve accessibility and color contrast

I was previewing the new website on my phone and I noticed that the color contrast is quite low on the search form at the top. So I checked the whole front page with [Pa11y](https://github.com/pa11y/pa11y) and found out that there are quite a few errors in terms of accessibility.

The majority of them are quite easy to fix so it shouldn't take long to do that.


## A few errors

```
Error: The html element should have a lang or xml:lang attribute which describes the language of the document.
├── WCAG2AA.Principle3.Guideline3_1.3_1_1.H57.2
├── html
└──

• Error: Img element is the only content of the link, but is missing alt text. The alt text should describe the purpose of the link.
├── WCAG2AA.Principle1.Guideline1_1.1_1_1.H30.2
├── html > body > div:nth-child(1) > div > header > a
└──

• Error: Img element is the only content of the link, but is missing alt text. The alt text should describe the purpose of the link.
├── WCAG2AA.Principle1.Guideline1_1.1_1_1.H30.2
├── html > body > div:nth-child(2) > div > div:nth-child(1) > div > div > div > div:nth-child(3) > div > div > div > div > a:nth-child(1)
└──

Start by running Pa11y against the front page and compare its output with the listed selectors, including #search_input_react, #hero-repl-in, and #hero-repl-out. Review the affected page elements for the reported language, image-alt, form-label, textarea-name, and color-contrast errors; done means the reported accessibility errors are resolved.

Written by the indexing model from the issue text.