UI: Theme extensions (#4398)

* Theme extensions

Adds the ability to choose the themeing strategy: Extend one of the existing themes (light or dark) or go fully custom. The latter was the only option up to now, which isn't ideal:

- One had to provide a full-blown theme file overriding all variables
- Tedious, error prone and hard to maintain, because one has to keep track of updates

This PR makes it so that one can choose light or dark as base theme and do modifications on top.

Benefit: You can specify a limited set of variables and might get away with 5-20 lines of CSS.

* Ensure custom theme is present

* Update checkout test

BTCPayServer.Tests/SeleniumTester.cs

@@ -193,6 +193,8 @@ namespace BTCPayServer.Tests
             Driver.WaitForElement(By.Id("OnChainWithLnInvoiceFallback"));
             Driver.SetCheckbox(By.Id("OnChainWithLnInvoiceFallback"), bip21);
             Driver.FindElement(By.Id("Save")).Click();
+            Assert.Contains("Store successfully updated", FindAlertMessage().Text);
+            Assert.True(Driver.FindElement(By.Id("UseNewCheckout")).Selected);
         }
 
         public Mnemonic GenerateWallet(string cryptoCode = "BTC", string seed = "", bool? importkeys = null, bool isHotWallet = false, ScriptPubKeyType format = ScriptPubKeyType.Segwit)