Frequently asked questions regarding theming in OpenUI5
While there is always the option to create a new theme, this is overkill for most minor style adaptations. For those minor changes, the recommendation is to include additional CSS into the page which changes the style of the respective tags of the OpenUI5 control. This allows complete, arbitrary changes of the visual design - after all it is the same technology that the UI5 controls use for their styling.
The main options are the following:
.addStyleClass("myStyle") on some control instances if you want only those instances to look different from other instances - and then write CSS code that refers to the normal classes/tags and to the CSS class you just added.Note:
- With this high degree of power and flexibility comes quite some responsibility. With CSS you can easily break the functionality of a control. This is not OpenUI5-specific, but when you make CSS adaptions, you should always have good knowledge of this open standard.
- The inner structure of a control, the tag hierarchy, the IDs and CSS classes are not part of the public control API for which we guarantee stability. This is also the case for other UI libraries which might define some CSS classes as stable, but not everything else. As CSS can refer to the inner structures of a control, you have to accept the risk that your style changes break when we change the inner structure. Changing the inner structure is a freedom we absolutely need to reserve, so we can fix bugs and add features of a control.
- When your CSS does not work as expected, use the developer tools in your browser to inspect the page and check which CSS rules are applied to the respective tag, and which rules might be applied but are overridden by other rules. If your rules are overridden by other rules, this is probably due to their order of appearance (last rule wins) or the CSS selector specificity (more specific CSS selectors win).
DON’Ts
When OpenUI5 is used in a standard way, which means loaded by a<script> element in the <head> of a page, and all libraries declared in the respective attribute of the script tag), it is sufficient to just add the custom CSS to any place after the OpenUI5 <script> element. OpenUI5 will insert its CSS links immediately after the <script> tag, so any subsequent CSS will appear further down in the DOM and can thus overwrite the OpenUI5 CSS.
However, it is important to understand the precedence rules of CSS: The order of appearance is not the only factor that determines which one of two or more conflicting rules wins. Actually it is only the least important factor. The most important (and maybe least known) factor is the specificity of the selector belonging to a rule.
For example, if one rule says button {color:red;} to make all button texts red, and a second rule says div > button {color:green;} to make all button texts, which are direct children of a <div> element, green, the second rule always wins because it is more specific. The order of appearance in the DOM does not matter in this case. It would only matter if both rules started with an equal selector, such as button{color:***}.
The order of loading is completely irrelevant, only the position in the DOM counts in this case. If you load OpenUI5 without a <script> tag in the <head>, or if you do not specify all used control libraries in the <script> tag, but loaded some of them later on when the body was already loaded, you can still make sure a custom CSS appears further down in the DOM by loading it with sap.ui.dom.includeStyleSheet(stylesheetUrl[, id])after loading OpenUI5 or the dynamically loaded control library.
Related Information
style property where I can make arbitrary changes?A control usually does not map to one HTML element, but to a whole tree of HTML elements. Whatever is set for the style property would probably be added to the root element of this HTML tree, and only there, so there is no style access to inner parts. If you just want to override the height of a button, this would actually work. But as soon as a change is a bit more complex, it will not work that easily. A more complex change is, for example, adapting the height of a ComboBox control. The outer <div> will get the proper height. And incidentally also the <input> tag inside, as it has 100% height set. But the dropdown arrow and the respective button-kind-of-thing has a fixed height, and the whole control will look pretty broken then.
In other cases, when HTML elements that break the CSS inheritance chain are nested, for example, <table> and font settings, you can change style to a different font and text color, but it will simply do nothing.
In general, we try to expose the obvious adaptation content in the API, for example, the button height. But the less obvious adaptations might have to be supported from inside the control to work properly, and as we cannot foresee and support everything you can do with a style property, we raise the bar a little bit by requiring you to write CSS (potentially using .addStyleClass(…) for the respective control). With CSS you can do what you cannot do with a style property: tweak the inner HTML components of a control.
Applications (at least the more traditional ones – currently this seems to be less of a rule, but I’m not sure it will stay like this forever) need to conform to some visual design guideline and, in general, it is not even desired that applications change the TextField height or use font just the way they like. As you can use CSS, UI5 still supports that, but we shouldn’t make breaking the visual design a rule in our official API.
If you want to change some styling and use control.addStyleClass(…) to add a CSS class, but it does not seem to work, you first have to pin down exactly what is not working:
You can check this by inspecting the HTML with your browser’s developer tools.
sap.ui.core.Element). Only some of them support addStyleClass.On all OpenUI5 application pages, the HTML root tag of the DOM gets the additional attribute data-sap-ui-browser where the value is the type and the current browser version. When browser-specific CSS needs to be written, this attribute can be used in CSS selectors.
html[data-sap-ui-browser*="sf"] button { /* this rule will only be applied if the current browser is ANY version of Safari */
padding-top: 0px;
}
There is not one single way to create a new theme, but there are several options. Which one you choose depends on several factors:
Depending on the answers it might be better to not even create a new theme but just adapt an existing one.