94485: Use relative path for theme CSS

This bug was originally fixed in DSpace#1642 but reintroduced in DSpace#1805

Author: ybnd

src/app/shared/theme-support/theme.service.spec.ts

@@ -413,7 +413,7 @@ describe('ThemeService', () => {
       link.setAttribute('rel', 'stylesheet');
       link.setAttribute('type', 'text/css');
       link.setAttribute('class', 'theme-css');
-      link.setAttribute('href', '/custom-theme.css');
+      link.setAttribute('href', 'custom-theme.css');
 
       expect(headSpy.appendChild).toHaveBeenCalledWith(link);
     });

src/app/shared/theme-support/theme.service.ts

@@ -182,7 +182,7 @@ export class ThemeService {
     link.setAttribute('rel', 'stylesheet');
     link.setAttribute('type', 'text/css');
     link.setAttribute('class', 'theme-css');
-    link.setAttribute('href', `/${encodeURIComponent(themeName)}-theme.css`);
+    link.setAttribute('href', `${encodeURIComponent(themeName)}-theme.css`);
     // wait for the new css to download before removing the old one to prevent a
     // flash of unstyled content
     link.onload = () => {