[PATCH proxmox-backup 1/2] docs: fix visited link contrast in dark mode

[Shannon Sterz <s.sterz@proxmox.com>]

by choosing a much lighter purple than the default visited link color
as secondary color. this increases the contrast from 1.37 to 7.08.
meaning, the text now meets WCAG AAA conditions for accessibility. the
light theme is unaffected.

Signed-off-by: Shannon Sterz <s.sterz@proxmox.com>
---
 docs/_static/custom.css | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/docs/_static/custom.css b/docs/_static/custom.css
index 7022b532b..5aeef4ce3 100644
--- a/docs/_static/custom.css
+++ b/docs/_static/custom.css
@@ -98,6 +98,7 @@ div.document, div.footer {
         --pdt-grey-100: hsl(0deg, 0%, 10%);
         --pdt-primary-800: hsl(205deg, 100%, 80%);
         --pdt-primary-700: hsl(205deg, 100%, 70%);
+        --pdt-secondary-800: hsl(275deg, 100%, 80%);
         --pdt-red-200: hsl(360deg, 30%, 20%);
         --pdt-red-350: hsl(360deg, 30%, 35%);
         --pdt-yellow-250: hsl(40deg, 100%, 25%);
@@ -107,6 +108,7 @@ div.document, div.footer {
         --pdt-shadow: var(--pdt-grey-100);
         --pdt-headline: var(--pdt-primary-800);
         --pdt-link: var(--pdt-primary-700);
+        --pdt-link-visited: var(--pdt-secondary-800);
         --pdt-admonition-background: var(--pdt-grey-250);
         --pdt-topic-background: var(--pdt-grey-200);
         --pdt-warn: var(--pdt-yellow-250);
@@ -136,6 +138,10 @@ div.document, div.footer {
         background-color: var(--pdt-admonition-background)
     }
 
+    a:visited {
+        color: var(--pdt-link-visited);
+    }
+
     div.sphinxsidebar ul ul a {
         color: var(--pdt-text);
         border-bottom-color: var(--pdt-border);
-- 
2.47.3