Introduce darkmode theme (#1969)

This commit introduces a new more relaxed theme, the dark theme!
In addition, the theme can now be selected according to the system defaults.
And the other themes, as well as the general structure were unified/simplified.

Co-authored-by: Philipp Ahrendt <philipp.ahrendt@cloudogu.com>
Co-authored-by: Eduard Heimbuch <eduard.heimbuch@cloudogu.com>
Co-authored-by: René Pfeuffer <rene.pfeuffer@cloudogu.com>
Co-authored-by: Matthias Thieroff <matthias.thieroff@cloudogu.com>

scm-ui/ui-webapp/src/repos/components/form/RepositoryFormSwitcher.tsx

@@ -45,7 +45,7 @@ const RepositoryFormButton: FC<RepositoryForm> = ({ path, icon, label }) => {
       color={isSelected ? "link is-selected" : undefined}
       link={!isSelected ? href : undefined}
     >
-      <Icon className="pr-2" name={icon} color={isSelected ? "secondary-least" : "inherit"} alt="" />
+      <Icon className="pr-2" name={icon} color="inherit" alt="" />
       <p className={classNames("is-hidden-mobile", "is-hidden-tablet-only")}>{t(`plugins:${label}`, label)}</p>
     </Button>
   );