[Accessibility] Increase of color contrast on parts of site

[hostle83]

Describe the bug

According to WCAG 2.0, color contrast for large text (defined as 14 pt and bold or larger, or 18 point or larger) should at least be 3:1 (4:5:1 for normal text). Parts of the current site fail a color contrast check.

• On https://demo.dspace.org/mydspace?configuration=workspace: "Search the repository", "10" (under "Results per page"), and "Last modified Descending" (Under "Sort By")

• On https://demo.dspace.org/browse/srsc: "Filter results by typing the first few letters", "Search", and "Add"

• On https://demo.dspace.org/browse/subject: "Search the repository" and "Filter results by typing the first few letters" (small text below the search box)

• Also see screenshot:

To Reproduce

Steps to reproduce the behavior:
I used the Silktide plugin on demo.dspace.org to check color contrasts on each site.

Expected behavior

Increase color contrast to at least 3:1 for large text. Ideally (level AAA), to 4:5:1. See also https://webaim.org/resources/contrastchecker/