Fix css calc expression to have space around operator

[tsmaeder]

What it does

Fixes #14204

Contributed on behalf of STMicroelectronics

How to test

Make sure toolbar items look O.K. in browser and electron cases

Follow-ups

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

I can confirm that the doesn't lead to any regressions. In fact, it doesn't do anything, since the rule in questions is always overriden:

See #3864 and the code:

theia/packages/core/src/browser/style/sidepanel.css

Lines 351 to 355 in 7ea9c13

	.theia-sidepanel-toolbar .p-TabBar-toolbar .item > div {
	height: 18px;
	width: 18px;
	background-repeat: no-repeat;
	}

Maybe we should remove the height and width attributes above?

[tsmaeder]

The fact that we're overriding the rule does not mean that adopters are not using it. This PR is just to fix the syntax issue, I haven't really thought about the larger picture.